[web] Add BackgroundIsolateBinaryMessenger.ensureInitialized to web. (#145786)

This PR makes the static API of the `BackgroundIsolateBinaryMessenger` consistent between its web and IO implementations.

This way, multi-platform applications will compile for the web when using the `ensureInitialized` method (even though it'll throw at runtime, like the `instance` getter, because isolates are not supported on the web).

### Issues

* Fixes: https://github.com/flutter/flutter/issues/145260

dev/bots/test.dart

@@ -598,6 +598,12 @@ Future<void> _runBuildTests() async {
             path.join('dev', 'integration_tests', 'web_compile_tests'),
             path.join('lib', 'dart_io_import.dart'),
           ),
+      // Should be able to compile with a call to:
+      // BackgroundIsolateBinaryMessenger.ensureInitialized.
+      () => _flutterBuildDart2js(
+            path.join('dev', 'integration_tests', 'web_compile_tests'),
+            path.join('lib', 'background_isolate_binary_messenger.dart'),
+          ),
     ],
     runForbiddenFromReleaseTests,
   ]..shuffle(math.Random(0));

dev/integration_tests/web_compile_tests/lib/background_isolate_binary_messenger.dart

+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'dart:ui' as ui;
+import 'package:flutter/services.dart';
+
+void main() {
+  BackgroundIsolateBinaryMessenger.ensureInitialized(
+    ui.RootIsolateToken.instance!
+  );
+}

packages/flutter/lib/src/services/_background_isolate_binary_messenger_web.dart

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'dart:ui' as ui show RootIsolateToken;
 import 'binding.dart';
 
 /// Stand-in for non-web platforms' [BackgroundIsolateBinaryMessenger].
@@ -10,4 +11,9 @@ class BackgroundIsolateBinaryMessenger {
   static BinaryMessenger get instance {
     throw UnsupportedError('Isolates not supported on web.');
   }
+
+  /// Throws an [UnsupportedError].
+  static void ensureInitialized(ui.RootIsolateToken token) {
+    throw UnsupportedError('Isolates not supported on web.');
+  }
 }