[web] UnimplementedError when using SliderTheme

## Steps to Reproduce



1. Run `flutter create --description "SliderThemeData UnimplementedError Web" --project-name slider_themedata_bug --platforms web,macos --org com.skalio.bug slider_themedata_bug`.
2. Update the file `lib/main.dart` as follows:

```
import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Slider ThemeData bug Web',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'This works for macos but not for web'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {

  double _currentSliderValue = 1;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: SliderTheme(
          data: SliderThemeData(
            trackShape: CustomTrackShape(),
            activeTrackColor: Colors.red,
            inactiveTrackColor: Colors.grey,
            thumbColor: Colors.blue,
            activeTickMarkColor: Colors.white,
            inactiveTickMarkColor: Colors.white,
          ),
          child: ShaderMask(
            blendMode: BlendMode.modulate,
            shaderCallback: (Rect bounds) {
              return LinearGradient(
                colors: <Color>[Colors.blue[50], Colors.blue],
                begin: Alignment.centerLeft,
                end: Alignment.centerRight,
                tileMode: TileMode.clamp,
              ).createShader(bounds);
            },
            child: Slider(
              onChanged: (double value) {
                      setState(() {
                        _currentSliderValue = value;
                      });
                    },
              min: 1,
              max: 3,
              value: _currentSliderValue,
              divisions: 2,
            ),
          ),
        ),
    ),);
  }

}

/// A TrackShape that can be Used for a [Slider] to allow for a wider,
/// margin-less Slider.
class CustomTrackShape extends RoundedRectSliderTrackShape {
  double margin = 10;
  Rect getPreferredRect({
    @required RenderBox parentBox,
    Offset offset = Offset.zero,
    @required SliderThemeData sliderTheme,
    bool isEnabled = false,
    bool isDiscrete = false,
  }) {
    final double trackHeight = sliderTheme.trackHeight;
    final double trackLeft = offset.dx + margin;
    final double trackTop = offset.dy + (parentBox.size.height - trackHeight) / 2;
    final double trackWidth = parentBox.size.width - 2 * margin;
    return Rect.fromLTWH(trackLeft, trackTop, trackWidth, trackHeight);
  }
}
```



3. Run example for web: `flutter run -d chrome`

**Expected results:** A nice slider styled with a SliderTheme

**Actual results:** Nothing is shown in the browser. Instead an `UnimplementedError` was thrown:

4. Run example for macOS: `flutter run -d macos` - The slider example works on macOS!

When showing the Slider without the SliderTheme it works on web.

**Please support SliderTheme for the web platform!**


<details>
  <summary>Logs</summary>

```
porcupine:slider_themedata_bug tom$ flutter run -d chrome --verbose
[  +71 ms] executing: [/usr/local/flutter/] git -c log.showSignature=false log -n 1 --pretty=format:%H
[  +45 ms] Exit code 0 from: git -c log.showSignature=false log -n 1 --pretty=format:%H
[   +2 ms] a77388e670ad9860c0be0d249c9c832de719e6f2
[        ] executing: [/usr/local/flutter/] git tag --points-at a77388e670ad9860c0be0d249c9c832de719e6f2
[  +73 ms] Exit code 0 from: git tag --points-at a77388e670ad9860c0be0d249c9c832de719e6f2
[   +1 ms] executing: [/usr/local/flutter/] git describe --match *.*.* --long --tags a77388e670ad9860c0be0d249c9c832de719e6f2
[  +76 ms] Exit code 0 from: git describe --match *.*.* --long --tags a77388e670ad9860c0be0d249c9c832de719e6f2
[        ] 1.26.0-17.0.pre-210-ga77388e670
[  +43 ms] executing: [/usr/local/flutter/] git rev-parse --abbrev-ref --symbolic @{u}
[  +14 ms] Exit code 0 from: git rev-parse --abbrev-ref --symbolic @{u}
[        ] origin/master
[        ] executing: [/usr/local/flutter/] git ls-remote --get-url origin
[  +13 ms] Exit code 0 from: git ls-remote --get-url origin
[        ] https://github.com/flutter/flutter.git
[  +51 ms] executing: [/usr/local/flutter/] git rev-parse --abbrev-ref HEAD
[  +14 ms] Exit code 0 from: git rev-parse --abbrev-ref HEAD
[        ] master
[   +6 ms] executing: sw_vers -productName
[  +12 ms] Exit code 0 from: sw_vers -productName
[        ] macOS
[        ] executing: sw_vers -productVersion
[  +13 ms] Exit code 0 from: sw_vers -productVersion
[        ] 11.2
[        ] executing: sw_vers -buildVersion
[  +15 ms] Exit code 0 from: sw_vers -buildVersion
[        ] 20D64
[   +5 ms] executing: sysctl hw.optional.arm64
[   +4 ms] Exit code 1 from: sysctl hw.optional.arm64
[        ] sysctl: unknown oid 'hw.optional.arm64'
[  +57 ms] 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.
[        ] Artifact Instance of 'FlutterWebSdk' 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.
[        ] Artifact Instance of 'FlutterRunnerSDKArtifacts' is not required, skipping update.
[        ] Artifact Instance of 'FlutterRunnerDebugSymbols' is not required, skipping update.
[  +42 ms] executing: /Users/tom/Library/Android/sdk/platform-tools/adb devices -l
[  +13 ms] executing: sysctl hw.optional.arm64
[   +4 ms] Exit code 1 from: sysctl hw.optional.arm64
[        ] sysctl: unknown oid 'hw.optional.arm64'
[        ] executing: xcrun xcodebuild -version
[ +134 ms] Exit code 0 from: xcrun xcodebuild -version
[   +2 ms] Xcode 12.4
           Build version 12D4e
[   +2 ms] executing: xcrun xcdevice list --timeout 2
[   +3 ms] xcrun simctl list --json devices
[        ] executing: xcrun simctl list --json devices
[  +12 ms] 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.
[   +1 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.
[  +56 ms] executing: /Users/tom/Library/Android/sdk/platform-tools/adb devices -l
[        ] executing: xcrun xcdevice list --timeout 2
[   +1 ms] xcrun simctl list --json devices
[        ] executing: xcrun simctl list --json devices
[  +13 ms] Skipping pub get: version match.
[ +119 ms] Generating /Users/tom/Development/slider_themedata_bug/android/app/src/main/java/io/flutter/plugins/GeneratedPluginRegistrant.java
[  +90 ms] List of devices attached
[   +2 ms] List of devices attached
[  +95 ms] Launching lib/main.dart on Chrome in debug mode...
[ +113 ms] {
                      "devices" : {

REMOVED DEVICES OUTPUT FOR BREVITY!

                 }
               ]
             }
           }
[  +36 ms] Updating assets
[  +79 ms] Waiting for connection from debug service on Chrome...
[  +63 ms] <- reset
[   +6 ms] /usr/local/flutter/bin/cache/dart-sdk/bin/dart --disable-dart-dev /usr/local/flutter/bin/cache/artifacts/engine/darwin-x64/frontend_server.dart.snapshot --sdk-root
/usr/local/flutter/bin/cache/flutter_web_sdk/ --incremental --target=dartdevc --debugger-module-names --experimental-emit-debug-metadata -DFLUTTER_WEB_AUTO_DETECT=true --output-dill
/var/folders/xb/0dzpwyg97cn7k1vx9nk3s73m0000gn/T/flutter_tools.ArVhqh/flutter_tool.zBqpnE/app.dill --libraries-spec file:///usr/local/flutter/bin/cache/flutter_web_sdk/libraries.json --packages
/Users/tom/Development/slider_themedata_bug/.dart_tool/package_config.json -Ddart.vm.profile=false -Ddart.vm.product=false --enable-asserts --track-widget-creation --filesystem-root
/var/folders/xb/0dzpwyg97cn7k1vx9nk3s73m0000gn/T/flutter_tools.ArVhqh/flutter_tools.ujzMDo --filesystem-scheme org-dartlang-app --initialize-from-dill build/c7922d95bf4a2462b75c84a97c312edb.cache.dill.track.dill
--platform file:///usr/local/flutter/bin/cache/flutter_web_sdk/kernel/flutter_ddc_sdk.dill --no-sound-null-safety
[  +17 ms] <- compile org-dartlang-app:/web_entrypoint.dart
[+2444 ms] [
                      {

REMOVED DEVICES OUTPUT FOR BREVIITY 

   
           ]
[+11894 ms] Waiting for connection from debug service on Chrome... (completed in 14,4s)
[        ] Synced 29.2MB.
[        ] <- accept
[        ] Caching compiled dill
[  +73 ms] Using Google Chrome 88.0.4324.146 

[ +537 ms] [CHROME]:
[        ] [CHROME]:DevTools listening on ws://127.0.0.1:50026/devtools/browser/22c72890-0df6-48ba-b3ff-4a0ff0950f43
[ +561 ms] DwdsInjector: Received request for entrypoint at http://localhost:49953/main_module.bootstrap.js
[   +5 ms] MetadataProvider: Loading debug metadata...
[   +8 ms] MetadataProvider: Loaded debug metadata (no sound null safety)
[   +7 ms] DwdsInjector: Injected debugging metadata for entrypoint at http://localhost:49953/main_module.bootstrap.js
[+1240 ms] ChromeProxyService: Initializing expression compiler for main_module.bootstrap.js with sound null safety: false
[ +301 ms] DevHandler: Debug service listening on ws://127.0.0.1:50052/Yl0j5_Mmt2g=/ws

[   +7 ms] Debug service listening on ws://127.0.0.1:50052/Yl0j5_Mmt2g=/ws
[        ] Running with unsound null safety
[        ] For more information see https://dart.dev/null-safety/unsound-null-safety
[   +1 ms] 🔥  To hot restart changes while running, press "r" or "R".
[        ] For a more detailed help message, press "h". To quit, press "q".
[+1352 ms] ══╡ EXCEPTION CAUGHT BY SCHEDULER LIBRARY ╞═════════════════════════════════════════════════════════
[   +3 ms] The following UnimplementedError was thrown during a scheduler callback:
[        ] UnimplementedError
[        ] When the exception was thrown, this was the stack:
[        ] dart-sdk/lib/_internal/js_dev_runtime/private/ddc_runtime/errors.dart 236:49  throw_
[        ] lib/_engine/engine/canvaskit/layer_scene_builder.dart 196:5                   pushShaderMask
[        ] packages/flutter/src/rendering/layer.dart 1779:26                             addToScene
[        ] packages/flutter/src/rendering/layer.dart 407:5                               [_addToSceneWithRetainedRendering]
[        ] packages/flutter/src/rendering/layer.dart 994:14                              addChildrenToScene
[        ] packages/flutter/src/rendering/layer.dart 1966:5                              addToScene
[        ] packages/flutter/src/rendering/layer.dart 407:5                               [_addToSceneWithRetainedRendering]
[        ] packages/flutter/src/rendering/layer.dart 994:14                              addChildrenToScene
[        ] packages/flutter/src/rendering/layer.dart 1131:5                              addToScene
[        ] packages/flutter/src/rendering/layer.dart 407:5                               [_addToSceneWithRetainedRendering]
[        ] packages/flutter/src/rendering/layer.dart 994:14                              addChildrenToScene
[        ] packages/flutter/src/rendering/layer.dart 1131:5                              addToScene
[        ] packages/flutter/src/rendering/layer.dart 407:5                               [_addToSceneWithRetainedRendering]
[        ] packages/flutter/src/rendering/layer.dart 994:14                              addChildrenToScene
[        ] packages/flutter/src/rendering/layer.dart 1576:5                              addToScene
[        ] packages/flutter/src/rendering/layer.dart 732:5                               buildScene
[   +1 ms] packages/flutter/src/rendering/view.dart 217:30                               compositeFrame
[        ] packages/flutter/src/rendering/binding.dart 457:18                            drawFrame
[        ] packages/flutter/src/widgets/binding.dart 874:13                              drawFrame
[        ] packages/flutter/src/rendering/binding.dart 319:5                             [_handlePersistentFrameCallback]
[        ] packages/flutter/src/scheduler/binding.dart 1144:15                           [_invokeFrameCallback]
[        ] packages/flutter/src/scheduler/binding.dart 1082:9                            handleDrawFrame
[        ] packages/flutter/src/scheduler/binding.dart 865:7                             <fn>
[        ] dart-sdk/lib/_internal/js_dev_runtime/private/isolate_helper.dart 48:19       internalCallback
[        ] ════════════════════════════════════════════════════════════════════════════════════════════════════
[        ] Another exception was thrown: UnimplementedError
[+5378 ms] Another exception was thrown: UnimplementedError
[   +5 ms] Another exception was thrown: UnimplementedError
[   +2 ms] Another exception was thrown: UnimplementedError
[  +16 ms] Another exception was thrown: UnimplementedError
[  +16 ms] Another exception was thrown: UnimplementedError
```


```
porcupine:slider_themedata_bug tom$ flutter analyze
Analyzing slider_themedata_bug...                                       
No issues found! (ran in 1.6s)
porcupine:slider_themedata_bug tom$ 
```


```
porcupine:slider_themedata_bug tom$ flutter doctor -v
[✓] Flutter (Channel master, 1.26.0-18.0.pre.210, on macOS 11.2 20D64 darwin-x64, locale de-DE)
    • Flutter version 1.26.0-18.0.pre.210 at /usr/local/flutter
    • Framework revision a77388e670 (3 hours ago), 2021-02-04 22:51:04 -0800
    • Engine revision b04955656c
    • Dart version 2.13.0 (build 2.13.0-0.0.dev)

[!] Android toolchain - develop for Android devices (Android SDK version 29.0.0)
    • Android SDK at /Users/tom/Library/Android/sdk
    • Platform android-29, build-tools 29.0.0
    • 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-1343-b01)
    ✗ Android license status unknown.
      Run `flutter doctor --android-licenses` to accept the SDK licenses.
      See https://flutter.dev/docs/get-started/install/macos#android-setup for more details.

[✓] Xcode - develop for iOS and macOS
    • Xcode at /Applications/Xcode.app/Contents/Developer
    • Xcode 12.4, Build version 12D4e
    • CocoaPods version 1.10.1

[✓] Chrome - develop for the web
    • Chrome at /Applications/Google Chrome.app/Contents/MacOS/Google Chrome

[✓] Android Studio (version 3.4)
    • Android Studio at /Applications/Android Studio.app/Contents
    • Flutter plugin can be installed from:
      🔨 https://plugins.jetbrains.com/plugin/9212-flutter
    • Dart plugin can be installed from:
      🔨 https://plugins.jetbrains.com/plugin/6351-dart
    • Java version OpenJDK Runtime Environment (build 1.8.0_152-release-1343-b01)

[✓] VS Code (version 1.53.0)
    • VS Code at /Applications/Visual Studio Code.app/Contents
    • Flutter extension version 3.19.0

[✓] Connected device (2 available)
    • macOS (desktop) • macos  • darwin-x64     • macOS 11.2 20D64 darwin-x64
    • Chrome (web)    • chrome • web-javascript • Google Chrome 88.0.4324.146

! Doctor found issues in 1 category.
```

</details>

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[web] UnimplementedError when using SliderTheme #75495

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[web] UnimplementedError when using SliderTheme #75495

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