Repository: flutter
Issue: findsOneWidget erroneously passes for DropdownMenuItem that is off screen under DropdownButton.
Labels: a: tests,framework,f: material design,has reproducible steps,P2,found in release: 3.3,found in release: 3.7,team-design,triaged-design

## Steps to Reproduce
* Create a Mobile app project for Flutter in Android Studio
* Create a unit test in a new my_test.dart file:

```
import 'package:flutter/material.dart';
import 'package:flutter_test/flutter_test.dart';

Future void main() async {
  testWidgets('drop down value', (tester) async {
      final Widget myWidget = DropdownButtonHideUnderline(
        child: DropdownButton<String>(
          isExpanded: true,
          value: '123', // ********* Actual selected value visible on screen, '123'
          onChanged: (_) => print(_),
          items: ['123', '456', '789'].map( // ********* Creating list of values in drop down: 123, 456, 789
              (value) {
              return DropdownMenuItem(
                value: value,
                child: Text(value?.toString() ?? 'hint'),
              );
            },
          ).toList(),
        ),
      );
      await pumpMaterial(tester, myWidget);

      expect(find.text('123'), findsOneWidget); // ********* Passes
      expect(find.text('456'), findsOneWidget); // ********* Passes
      expect(find.text('789'), findsOneWidget); // ********* Passes
    });
}

Future<void> pumpMaterial(WidgetTester tester, Widget widget) async {
  final app = MaterialApp(
    home: Material(
      child: Center(
        child: widget,
      ),
    ),
  );

  await tester.pumpWidget(app);
}
```

* Run the test.
* **Actual result:**
All 3 "expect" checks pass, the test passes and shows up green.
* **Expected result:**
Only the check for string '123' should pass, others should fail, as their text is not visible on the screen.

## Logs

Log:
```
Testing started at 8:49 AM ...
/Users/acherkashyn/Flutter/flutter/bin/flutter --no-color test --machine --plain-name "drop down value" test/my_test.dart
```

flutter analyze:
```
Analyzing app...                                                        
No issues found! (ran in 5.0s)
```

flutter doctor -v:
```
[✓] Flutter (Channel stable, v1.2.1, on Mac OS X 10.14.3 18D42, locale en-US)
    • Flutter version 1.2.1 at /Users/acherkashyn/Flutter/flutter
    • Framework revision 8661d8aecd (4 weeks ago), 2019-02-14 19:19:53 -0800
    • Engine revision 3757390fa4
    • Dart version 2.1.2 (build 2.1.2-dev.0.0 0a7dcf17eb)

[✓] Android toolchain - develop for Android devices (Android SDK version 28.0.3)
    • Android SDK at /Users/acherkashyn/Library/Android/sdk
    • Android NDK location not configured (optional; useful for native profiling support)
    • Platform android-28, build-tools 28.0.3
    • ANDROID_HOME = /Users/acherkashyn/Library/Android/sdk
    • Java binary at: /Applications/Android Studio.app/Contents/jre/jdk/Contents/Home/bin/java
    • Java version OpenJDK Runtime Environment (build 1.8.0_152-release-1248-b01)
    • All Android licenses accepted.

[✓] iOS toolchain - develop for iOS devices (Xcode 10.1)
    • Xcode at /Applications/Xcode.app/Contents/Developer
    • Xcode 10.1, Build version 10B61
    • ios-deploy 1.9.4
    • CocoaPods version 1.6.1

[✓] Android Studio (version 3.3)
    • Android Studio at /Applications/Android Studio.app/Contents
    • Flutter plugin version 33.3.1
    • Dart plugin version 182.5215
    • Java version OpenJDK Runtime Environment (build 1.8.0_152-release-1248-b01)

[✓] IntelliJ IDEA Community Edition (version 2018.3.4)
    • IntelliJ at /Applications/IntelliJ IDEA CE.app
    • Flutter plugin version 32.0.3
    • Dart plugin version 183.5901

[✓] VS Code (version 1.31.1)
    • VS Code at /Applications/Visual Studio Code.app/Contents
    • Flutter extension version 2.23.0

[✓] Connected device (1 available)
    • iPhone XR • F6F6EA6E-9B25-44D7-AA86-0221686AE960 • ios • iOS 12.1 (simulator)

• No issues found!
```