[flutter_tools] Add web support through MethodChannels. (#60116)

packages/flutter_tools/templates/plugin/lib/projectName_web.dart.tmpl

@@ -0,0 +1,45 @@
+import 'dart:async';
+// In order to *not* need this ignore, consider extracting the "web" version
+// of your plugin as a separate package, instead of inlining it in the same
+// package as the core of your plugin.
+// ignore: avoid_web_libraries_in_flutter
+import 'dart:html' as html show window;
+
+import 'package:flutter/services.dart';
+import 'package:flutter_web_plugins/flutter_web_plugins.dart';
+
+/// A web implementation of the {{pluginDartClass}} plugin.
+class {{pluginDartClass}}Web {
+  static void registerWith(Registrar registrar) {
+    final MethodChannel channel = MethodChannel(
+      '{{projectName}}',
+      const StandardMethodCodec(),
+      registrar.messenger,
+    );
+
+    final pluginInstance = {{pluginDartClass}}Web();
+    channel.setMethodCallHandler(pluginInstance.handleMethodCall);
+  }
+
+  /// Handles method calls over the MethodChannel of this plugin.
+  /// Note: Check the "federated" architecture for a new way of doing this:
+  /// https://flutter.dev/go/federated-plugins
+  Future<dynamic> handleMethodCall(MethodCall call) async {
+    switch (call.method) {
+      case 'getPlatformVersion':
+        return getPlatformVersion();
+        break;
+      default:
+        throw PlatformException(
+          code: 'Unimplemented',
+          details: '{{projectName}} for web doesn\'t implement \'${call.method}\'',
+        );
+    }
+  }
+
+  /// Returns a [String] containing the version of the platform.
+  Future<String> getPlatformVersion() {
+    final version = html.window.navigator.userAgent;
+    return Future.value(version);
+  }
+}

packages/flutter_tools/templates/plugin/pubspec.yaml.tmpl

@@ -11,6 +11,10 @@ environment:
 dependencies:
   flutter:
     sdk: flutter
+{{#web}}
+  flutter_web_plugins:
+    sdk: flutter
+{{/web}}
 
 dev_dependencies:
   flutter_test:
@@ -40,6 +44,11 @@ flutter:
       macos:
         pluginClass: {{pluginClass}}
 {{/macos}}
+{{#web}}
+      web:
+        pluginClass: {{pluginDartClass}}Web
+        fileName: {{projectName}}_web.dart
+{{/web}}
 
   # To add assets to your plugin package, add an assets section, like this:
   # assets:

packages/flutter_tools/test/commands.shard/permeable/create_test.dart

@@ -391,6 +391,9 @@ void main() {
         'ios/Classes/FlutterProjectPlugin.m',
         'lib/flutter_project.dart',
       ],
+      unexpectedPaths: <String>[
+        'lib/flutter_project_web.dart',
+      ],
     );
     return _runFlutterTest(projectDir.childDirectory('example'));
   }, overrides: <Type, Generator>{
@@ -404,6 +407,34 @@ void main() {
     ),
   });
 
+  testUsingContext('plugin project supports web', () async {
+    await _createAndAnalyzeProject(
+      projectDir,
+      <String>['--template=plugin'],
+      <String>[
+        'lib/flutter_project.dart',
+        'lib/flutter_project_web.dart',
+      ],
+    );
+    final String rawPubspec = await projectDir.childFile('pubspec.yaml').readAsString();
+    final Pubspec pubspec = Pubspec.parse(rawPubspec);
+    // Expect the dependency on flutter_web_plugins exists
+    expect(pubspec.dependencies, contains('flutter_web_plugins'));
+    // The platform is correctly registered
+    expect(pubspec.flutter['plugin']['platforms']['web']['pluginClass'], 'FlutterProjectWeb');
+    expect(pubspec.flutter['plugin']['platforms']['web']['fileName'], 'flutter_project_web.dart');
+  }, overrides: <Type, Generator>{
+    FeatureFlags: () => TestFeatureFlags(isWebEnabled: true),
+    Pub: () => Pub(
+      fileSystem: globals.fs,
+      logger: globals.logger,
+      processManager: globals.processManager,
+      usage: globals.flutterUsage,
+      botDetector: globals.botDetector,
+      platform: globals.platform,
+    ),
+  });
+
   testUsingContext('plugin example app depends on plugin', () async {
     await _createProject(
       projectDir,