Repository: flutter
Issue: accessibility test meetsGuideline(textContrastGuideline) throws on web
Labels: a: tests,framework,a: accessibility,platform-web,P2,c: parity,c: tech-debt,team: skip-test,team-web,triaged-web

This issues surfaced in a pre-submit test: https://cirrus-ci.com/task/4889346271870976

### Repro code:
`flutter test --platform=chrome test/cupertino/button_test.dart`
```dart
      await tester.pumpWidget(
        CupertinoApp(
          theme: const CupertinoThemeData(brightness: Brightness.dark),
          home: CupertinoPageScaffold(
            child: CupertinoButton.filled(
              child: const Text('Button'),
              onPressed: () {},
            ),
          )
        ),
      );

      await expectLater(tester, meetsGuideline(textContrastGuideline));
```

### Stacktrace
```
══╡ EXCEPTION CAUGHT BY FLUTTER TEST FRAMEWORK ╞════════════════════════════════════════════════════                                                                                                                                                                                                                  [85/1869]
The following _Exception was thrown while running async test code:                                                                                                                                                                                                                                                             
Exception: Incorrect sequence of push/pop operations while building scene surfaces. After building                                                                                                                                                                                                                             
the scene the persisted surface stack must contain a single element which corresponds to the scene                                                                                                                                                                                                                             
itself (_PersistedScene). All other surfaces should have been popped off the stack. Found the                                                                                                                                                                                                                                  
following surfaces in the stack:                                                                                                                                                                                                                                                                                               
PersistedScene, PersistedTransform                                                                                                                                                                                                                                                                                             
                                                                                                                                                                                                                                                                                                                               
When the exception was thrown, this was the stack:                                                                                                                                                                                                                                                                             
package:dart-sdk/lib/_internal/js_dev_runtime/private/ddc_runtime/errors.dart 194:49      throw_                                                                                                                                                                                                                               
package:build_web_compilers/lib/_engine/engine/surface/scene_builder.dart 26:9            <fn>                                                                                                                                                                                                                                 
package:build_web_compilers/lib/_engine/engine/surface/scene_builder.dart 33:14           get [_persistedScene]                                                                                                                                                                                                                
package:build_web_compilers/lib/_engine/engine/surface/scene_builder.dart 464:21          build                                                                                                                                                                                                                                
package:flutter/src/rendering/layer.dart 767:35                                           buildScene                                                                                                                                                                                                                           
package:flutter/src/rendering/layer.dart 1196:28                                          toImage                                                                                                                                                                                                                              
package:dart-sdk/lib/_internal/js_dev_runtime/patch/async_patch.dart 84:54                runBody                                                                                                                                                                                                                              
package:dart-sdk/lib/_internal/js_dev_runtime/patch/async_patch.dart 123:12               _async                                                                                                                                                                                                                               
package:flutter/src/rendering/layer.dart 1185:27                                          toImage                                                                                                                                                                                                                              
package:flutter_test/src/accessibility.dart 204:26                                        <fn>                                                                                                                                                                                                                                 
package:dart-sdk/lib/_internal/js_dev_runtime/patch/async_patch.dart 84:54                runBody                                                                                                                                                                                                                              
package:dart-sdk/lib/_internal/js_dev_runtime/patch/async_patch.dart 123:12               _async                                                                                                                                                                                                                               
package:flutter_test/src/accessibility.dart 201:71                                        <fn>                                                                                                                                                                                                                                 
package:flutter_test/src/binding.dart 923:10                                              <fn>                                                                                                                                                                                                                                 
package:dart-sdk/lib/async/zone.dart 1124:13                                              _rootRun                                                                                                                                                                                                                             
package:dart-sdk/lib/async/zone.dart 1021:19                                              run                                                                                                                                                                                                                                  
package:flutter_test/src/binding.dart 913:25                                              runAsync                                                                                                                                                                                                                             
package:flutter_test/src/accessibility.dart 201:43                                        evaluate                                                                                                                                                                                                                             
package:dart-sdk/lib/_internal/js_dev_runtime/patch/async_patch.dart 84:54                runBody                                                                                                                                                                                                                              
package:dart-sdk/lib/_internal/js_dev_runtime/patch/async_patch.dart 123:12               _async                                                                                                                                                                                                                               
package:flutter_test/src/accessibility.dart 196:30                                        evaluate                                                                                                                                                                                                                             
package:flutter_test/src/matchers.dart 1973:47                                            matchAsync                                                                                                                                                                                                                           
package:dart-sdk/lib/_internal/js_dev_runtime/patch/async_patch.dart 84:54                runBody                                                                                                                                                                                                                              
package:dart-sdk/lib/_internal/js_dev_runtime/patch/async_patch.dart 123:12               _async                                                                                                                                                                                                                               
package:flutter_test/src/matchers.dart 1972:28                                            matchAsync                                                                                                                                                                                                                           
package:test_api/src/frontend/expect.dart 116:25                                          _expect                                                                                                                                                                                                                              
package:test_api/src/frontend/expect.dart 75:5                                            expectLater                                                                                                                                                                                                                          
package:flutter_test/src/widget_tester.dart 269:13                                        expectLater                                                                                                                                                                                                                          
button_test.dart 100:13                                                                   <fn>                                                                                                                                                                                                                                 
package:dart-sdk/lib/_internal/js_dev_runtime/patch/async_patch.dart 45:50                onValue                                                                                                                                                                                                                              
package:stack_trace/src/stack_zone_specification.dart 129:26                              <fn>                                                                                                                                                                                                                                 
package:stack_trace/src/stack_zone_specification.dart 209:15                              [_run]                                                                                                                                                                                                                               
package:stack_trace/src/stack_zone_specification.dart 129:14                              <fn>                                                                                                                                                                                                                                 
package:dart-sdk/lib/async/zone.dart 1132:38                                              _rootRunUnary                                                                                                                                                                                                                        
package:dart-sdk/lib/async/zone.dart 1029:19                                              runUnary                                                                                                                                                                                                                             
package:dart-sdk/lib/async/future_impl.dart 137:18                                        handleValue                                                                                                                                                                                                                          
package:dart-sdk/lib/async/future_impl.dart 678:44                                        handleValueCallback                                                                                                                                                                                                                  
package:dart-sdk/lib/async/future_impl.dart 707:32                                        _propagateToListeners                                                                                                                                                                                                                
package:dart-sdk/lib/async/future_impl.dart 522:5                                         [_completeWithValue]                                                                                                                                                                                                                 
package:dart-sdk/lib/async/future_impl.dart 552:7                                         <fn>                                                                                                                                                                                                                                 
package:dart-sdk/lib/_internal/js_dev_runtime/private/ddc_runtime/operations.dart 324:14  _checkAndCall                                                                                                                                                                                                                        
package:dart-sdk/lib/_internal/js_dev_runtime/private/ddc_runtime/operations.dart 329:39  dcall                                                                                                                                                                                                                                
package:quiver/testing/src/async/fake_async.dart 268:32                                   [_drainMicrotasks]                                                                                                                                                                                                                   
package:quiver/testing/src/async/fake_async.dart 164:5                                    flushMicrotasks                                                                                                                                                                                                                      
package:flutter_test/src/binding.dart 1067:16                                             <fn>                                                                                                                                                                                                                                 
package:dart-sdk/lib/_internal/js_dev_runtime/patch/async_patch.dart 84:54                runBody                                                                                                                                                                                                                              
package:dart-sdk/lib/_internal/js_dev_runtime/patch/async_patch.dart 123:12               _async                                                                                                                                                                                                                               
package:flutter_test/src/binding.dart 1055:35                                             <fn>                                                                                                                                                                                                                                 
package:dart-sdk/lib/async/future.dart 200:37                                             <fn>                                                                                                                                                                                                                                 
package:stack_trace/src/stack_zone_specification.dart 209:15                              [_run]                                                                                                                                                                                                                               
package:stack_trace/src/stack_zone_specification.dart 119:48                              <fn>                                                                                                                                                                                                                                 
package:dart-sdk/lib/async/zone.dart 1120:38                                              _rootRun                                                                                                                                                                                                                             
package:dart-sdk/lib/async/zone.dart 1021:19                                              run                                                                                                                                                                                                                                  
package:dart-sdk/lib/async/zone.dart 923:7                                                runGuarded                                                                                                                                                                                                                           
package:dart-sdk/lib/async/zone.dart 963:23                                               <fn>                                                                                                                                                                                                                                 
package:stack_trace/src/stack_zone_specification.dart 209:15                              [_run]                                                                                                                                                                                                                               
package:stack_trace/src/stack_zone_specification.dart 119:48                              <fn>                                                                                                                                                                                                                                 
package:dart-sdk/lib/async/zone.dart 1124:13                                              _rootRun                                                                                                                                                                                                                             
package:dart-sdk/lib/async/zone.dart 1021:19                                              run                                                                                                                                                                                                                                  
package:dart-sdk/lib/async/zone.dart 923:7                                                runGuarded                                                                                                                                                                                                                           
package:dart-sdk/lib/async/zone.dart 963:23                                               callback                                                                                                                                                                                                                             
package:dart-sdk/lib/async/schedule_microtask.dart 41:11                                  _microtaskLoop                                                                                                                                                                                                                       
package:dart-sdk/lib/async/schedule_microtask.dart 50:5                                   _startMicrotaskLoop                                                                                                                                                                                                                  
package:dart-sdk/lib/_internal/js_dev_runtime/patch/async_patch.dart 166:15               <fn>   
```

### Version
```
[✓] Flutter (Channel unknown, v1.10.7-pre.107, on Linux, locale en_US.UTF-8)
    • Flutter version 1.10.7-pre.107 
    • Framework revision 2affe5ba7c (3 days ago), 2019-11-01 16:07:20 -0700
    • Engine revision 8ea19b1c76
    • Dart version 2.6.0 (build 2.6.0-dev.8.2 bbe2ac28c9)
```