[web] Fix `kIsWeb` for Dart2wasm. (#112122)

packages/flutter/lib/src/foundation/constants.dart

@@ -68,10 +68,4 @@ const bool kDebugMode = !kReleaseMode && !kProfileMode;
 const double precisionErrorTolerance = 1e-10;
 
 /// A constant that is true if the application was compiled to run on the web.
-///
-/// This implementation takes advantage of the fact that JavaScript does not
-/// support integers. In this environment, Dart's doubles and ints are
-/// backed by the same kind of object. Thus a double `0.0` is identical
-/// to an integer `0`. This is not true for Dart code running in AOT or on the
-/// VM.
-const bool kIsWeb = identical(0, 0.0);
+const bool kIsWeb = bool.fromEnvironment('dart.library.js_util');

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

@@ -8,7 +8,7 @@ import 'dart:developer';
 import 'package:flutter/foundation.dart';
 
 import '_background_isolate_binary_messenger_io.dart'
-  if (dart.library.html) '_background_isolate_binary_messenger_web.dart';
+  if (dart.library.js_util) '_background_isolate_binary_messenger_web.dart';
 
 import 'binary_messenger.dart';
 import 'binding.dart';
@@ -17,7 +17,7 @@ import 'message_codec.dart';
 import 'message_codecs.dart';
 
 export '_background_isolate_binary_messenger_io.dart'
-  if (dart.library.html) '_background_isolate_binary_messenger_web.dart';
+  if (dart.library.js_util) '_background_isolate_binary_messenger_web.dart';
 
 export 'binary_messenger.dart' show BinaryMessenger;
 export 'binding.dart' show RootIsolateToken;

packages/flutter/lib/src/widgets/platform_selectable_region_context_menu.dart

@@ -2,4 +2,5 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-export '_platform_selectable_region_context_menu_io.dart' if(dart.library.html) '_platform_selectable_region_context_menu_web.dart';
+export '_platform_selectable_region_context_menu_io.dart'
+if (dart.library.js_util) '_platform_selectable_region_context_menu_web.dart';

packages/flutter/test/dart/browser_environment_test.dart

@@ -11,7 +11,7 @@ import 'package:flutter_test/flutter_test.dart';
 // Originally here: https://github.com/flutter/engine/pull/28808
 void main() {
   test('Web library environment define exists', () {
-    expect(const bool.fromEnvironment('dart.library.html'), isTrue);
+    expect(const bool.fromEnvironment('dart.library.js_util'), isTrue);
     expect(const bool.fromEnvironment('dart.library.someFooLibrary'), isFalse);
   });
 }

packages/flutter/test/painting/network_image_web_test.dart

@@ -3,7 +3,7 @@
 // found in the LICENSE file.
 
 import '_network_image_web_test_io.dart'
-    if (dart.library.html) '_network_image_test_web.dart';
+    if (dart.library.js_util) '_network_image_test_web.dart';
 
 void main() {
   runTests();