Repository: flutter
Issue: Clipping not behaving correctly on web
Labels: engine,platform-web,c: rendering,has reproducible steps,P2,found in release: 3.3,found in release: 3.7,team-web,triaged-web

## Steps to Reproduce

1. Run `flutter create bug`.
2. Update the files as follows: 
 add the following to _incrementCounter():
```
showModalBottomSheet(
      shape: RoundedRectangleBorder(
          borderRadius: BorderRadius.vertical(top: Radius.circular(15))),
      context: context,
      builder: (ctx) => Container(
        width: double.infinity,
        height: 500,
        color: Colors.red,
      ),
    );
```

Next, change the background color of the scaffold to:
```
Scaffold(
      backgroundColor: Colors.blue, ...
```
3. Click on the button

**Expected results:** <!-- what did you want to see? -->
a nice, anti-aliased rounded border

**Actual results:** <!-- what did you see? -->
![imagen](https://user-images.githubusercontent.com/29983481/84566803-6b865a00-ad74-11ea-9820-9b88f6897af7.png)

<details>
  <summary>Logs</summary>


```
[   +9 ms] executing: [C:\flutter/] git -c log.showSignature=false log -n 1 --pretty=format:%H
[  +72 ms] Exit code 0 from: git -c log.showSignature=false log -n 1 --pretty=format:%H
[   +2 ms] f994b769743368b36b9c03fb359f62230b60ab92
[        ] executing: [C:\flutter/] git tag --contains HEAD
[ +203 ms] Exit code 0 from: git tag --contains HEAD
[   +1 ms] 1.19.0-4.1.pre
[   +9 ms] executing: [C:\flutter/] git rev-parse --abbrev-ref --symbolic @{u}
[  +55 ms] Exit code 0 from: git rev-parse --abbrev-ref --symbolic @{u}
[        ] origin/beta
[        ] executing: [C:\flutter/] git ls-remote --get-url origin
[  +42 ms] Exit code 0 from: git ls-remote --get-url origin
[   +1 ms] https://github.com/flutter/flutter.git
[  +81 ms] executing: [C:\flutter/] git rev-parse --abbrev-ref HEAD
[  +46 ms] Exit code 0 from: git rev-parse --abbrev-ref HEAD
[        ] beta
[  +51 ms] Artifact Instance of 'AndroidMavenArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'AndroidGenSnapshotArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'AndroidInternalBuildArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'IOSEngineArtifacts' is not required, skipping update.
[   +6 ms] Artifact Instance of 'FlutterWebSdk' is not required, skipping update.
[   +3 ms] Artifact Instance of 'WindowsEngineArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'MacOSEngineArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'LinuxEngineArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'LinuxFuchsiaSDKArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'MacOSFuchsiaSDKArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'FlutterRunnerSDKArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'FlutterRunnerDebugSymbols' is not required, skipping update.
[  +16 ms] executing: C:\Users\Aguilaair Dev\AppData\Local\Android\sdk\platform-tools\adb.exe devices -l
[  +38 ms] List of devices attached
[   +8 ms] Artifact Instance of 'AndroidMavenArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'AndroidGenSnapshotArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'AndroidInternalBuildArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'IOSEngineArtifacts' is not required, skipping update.
[   +2 ms] Artifact Instance of 'WindowsEngineArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'MacOSEngineArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'LinuxEngineArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'LinuxFuchsiaSDKArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'MacOSFuchsiaSDKArtifacts' is not required, skipping update.
[   +7 ms] Artifact Instance of 'FlutterRunnerSDKArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'FlutterRunnerDebugSymbols' is not required, skipping update.
[ +184 ms] Generating C:\Users\Aguilaair Dev\OneDrive\Escritorio\flutter 2\bug\android\app\src\main\java\io\flutter\plugins\GeneratedPluginRegistrant.java
[  +86 ms] Launching lib\main.dart on Chrome in debug mode...
[  +59 ms] Updating assets
[  +78 ms] Syncing files to device Chrome...
[  +42 ms] Generating C:\Users\Aguilaair Dev\OneDrive\Escritorio\flutter 2\bug\android\app\src\main\java\io\flutter\plugins\GeneratedPluginRegistrant.java
[  +39 ms] <- reset
[   +6 ms] C:\flutter\bin\cache\dart-sdk\bin\dart.exe --disable-dart-dev --old_gen_heap_size=2000 C:\flutter\bin\cache\artifacts\engine\windows-x64\frontend_server.dart.snapshot --sdk-root       
C:\flutter\bin\cache\flutter_web_sdk/ --incremental --target=dartdevc --debugger-module-names -Ddart.developer.causal_async_stacks=true --output-dill
C:\Users\AGUILA~1\AppData\Local\Temp\flutter_tools.6b06fbce\flutter_tool.c1d94085\app.dill --libraries-spec file:///C:/flutter/bin/cache/flutter_web_sdk/libraries.json --packages .packages       
-Ddart.vm.profile=false -Ddart.vm.product=false --bytecode-options=source-positions,local-var-info,debugger-stops,instance-field-initializers,keep-unreachable-code,avoid-closure-call-instructions--enable-asserts --track-widget-creation --filesystem-root C:\Users\AGUILA~1\AppData\Local\Temp\flutter_tools.6b06fbce\flutter_tools.27a2e977 --filesystem-scheme org-dartlang-app
--initialize-from-dill build\cache.dill.track.dill --platform file:///C:/flutter/bin/cache/flutter_web_sdk/kernel/flutter_ddc_sdk.dill
[  +11 ms] <- compile org-dartlang-app:/web_entrypoint.dart
[+10098 ms] Syncing files to device Chrome... (completed in 10.190ms,
```

<!--
     Run `flutter analyze` and attach any output of that command below.
     If there are any analysis errors, try resolving them before filing this issue.
-->

```
Analyzing bug...                                                        
No issues found! (ran in 1.5s)
```

<!-- Finally, paste the output of running `flutter doctor -v` here. -->

```
[√] Flutter (Channel beta, 1.19.0-4.1.pre, on Microsoft Windows [VersiÃ³n 10.0.19041.329], locale es-ES)
    • Flutter version 1.19.0-4.1.pre at C:\flutter
    • Framework revision f994b76974 (4 days ago), 2020-06-09 15:53:13 -0700
    • Engine revision 9a28c3bcf4
    • Dart version 2.9.0 (build 2.9.0-14.1.beta)

 
[√] Android toolchain - develop for Android devices (Android SDK version 29.0.3)
    • Android SDK at C:\Users\Aguilaair Dev\AppData\Local\Android\sdk
    • Platform android-29, build-tools 29.0.3
    • Java binary at: C:\Program Files\Android\Android Studio\jre\bin\java
    • Java version OpenJDK Runtime Environment (build 1.8.0_212-release-1586-b04)
    • All Android licenses accepted.

[√] Chrome - develop for the web
    • Chrome at C:\Program Files (x86)\Google\Chrome\Application\chrome.exe

[√] Android Studio (version 3.6)
    • Android Studio at C:\Program Files\Android\Android Studio
    • Flutter plugin version 45.1.1
    • Dart plugin version 192.7761
    • Java version OpenJDK Runtime Environment (build 1.8.0_212-release-1586-b04)

[√] VS Code (version 1.46.0)
    • VS Code at C:\Users\Aguilaair Dev\AppData\Local\Programs\Microsoft VS Code
    • Flutter extension version 3.11.0

[√] Connected device (2 available)
    • Web Server • web-server • web-javascript • Flutter Tools
    • Chrome     • chrome     • web-javascript • Google Chrome 83.0.4103.97

• No issues found!
```

</details>