[web] Use platform detection from Flutter web engine. (#147346)

> [!IMPORTANT]
> Requires the following engine PR:
> * https://github.com/flutter/engine/pull/52380
> ----

This PR refactors Flutter `foundation`'s library `platform` for the web with the same code we use to detect platforms in the engine.

## Issues

* Fixes: https://github.com/flutter/flutter/issues/128943

## Testing

Demo app deployed here:

* https://dit-browser-detect.web.app

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

@@ -4,7 +4,6 @@
 
 import 'dart:ui_web' as ui_web;
 
-import '../web.dart' as web;
 import 'platform.dart' as platform;
 
 export 'platform.dart' show TargetPlatform;
@@ -19,46 +18,45 @@ platform.TargetPlatform get defaultTargetPlatform {
       _browserPlatform;
 }
 
-final platform.TargetPlatform? _testPlatform = () {
-  platform.TargetPlatform? result;
+// The TargetPlatform used on Web tests, unless overridden.
+//
+// Respects the `ui_web.browser.debugOperatingSystemOverride` value (when set).
+platform.TargetPlatform? get _testPlatform {
+  platform.TargetPlatform? testPlatform;
   assert(() {
     if (ui_web.debugEmulateFlutterTesterEnvironment) {
-      result = platform.TargetPlatform.android;
+      // Return the overridden operatingSystem in tests, if any...
+      if (ui_web.browser.debugOperatingSystemOverride != null) {
+        testPlatform =
+          _operatingSystemToTargetPlatform(ui_web.browser.operatingSystem);
+      } else {
+        // Fall back to `android` for tests.
+        testPlatform = platform.TargetPlatform.android;
+      }
     }
     return true;
   }());
-  return result;
-}();
+  return testPlatform;
+}
 
-// Lazy-initialized and forever cached current browser platform.
+// Current browser platform.
 //
-// Computing the platform is expensive as it uses `window.matchMedia`, which
-// needs to parse and evaluate a CSS selector. On some devices this takes up to
-// 0.20ms. As `defaultTargetPlatform` is routinely called dozens of times per
-// frame this value should be cached.
-final platform.TargetPlatform _browserPlatform = () {
-  final String navigatorPlatform = web.window.navigator.platform.toLowerCase();
-  if (navigatorPlatform.startsWith('mac')) {
-    return platform.TargetPlatform.macOS;
-  }
-  if (navigatorPlatform.startsWith('win')) {
-    return platform.TargetPlatform.windows;
-  }
-  if (navigatorPlatform.contains('iphone') ||
-      navigatorPlatform.contains('ipad') ||
-      navigatorPlatform.contains('ipod')) {
-    return platform.TargetPlatform.iOS;
-  }
-  if (navigatorPlatform.contains('android')) {
-    return platform.TargetPlatform.android;
-  }
-  // Since some phones can report a window.navigator.platform as Linux, fall
-  // back to use CSS to disambiguate Android vs Linux desktop. If the CSS
-  // indicates that a device has a "fine pointer" (mouse) as the primary
-  // pointing device, then we'll assume desktop linux, and otherwise we'll
-  // assume Android.
-  if (web.window.matchMedia('only screen and (pointer: fine)').matches) {
-    return platform.TargetPlatform.linux;
-  }
-  return platform.TargetPlatform.android;
-}();
+// The computation of `operatingSystem` is cached in the ui_web package;
+// this getter may be called dozens of times per frame.
+//
+// _browserPlatform is lazily initialized, and cached forever.
+final platform.TargetPlatform _browserPlatform =
+  _operatingSystemToTargetPlatform(ui_web.browser.operatingSystem);
+
+// Converts an ui_web.OperatingSystem enum into a platform.TargetPlatform.
+platform.TargetPlatform _operatingSystemToTargetPlatform(ui_web.OperatingSystem os) {
+  return switch (os) {
+    ui_web.OperatingSystem.android => platform.TargetPlatform.android,
+    ui_web.OperatingSystem.iOs => platform.TargetPlatform.iOS,
+    ui_web.OperatingSystem.linux => platform.TargetPlatform.linux,
+    ui_web.OperatingSystem.macOs => platform.TargetPlatform.macOS,
+    ui_web.OperatingSystem.windows => platform.TargetPlatform.windows,
+    // Resolve 'unknown' OS values to `android`.
+    ui_web.OperatingSystem.unknown => platform.TargetPlatform.android,
+  };
+}

packages/flutter/test/foundation/platform_web_test.dart

@@ -0,0 +1,33 @@
+// 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.
+
+@TestOn('chrome')
+library;
+
+import 'dart:ui_web' as ui_web;
+
+import 'package:flutter/foundation.dart' show TargetPlatform, defaultTargetPlatform;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  tearDown(() {
+    // Remove the `debugOperatingSystemOverride`.
+    ui_web.browser.debugOperatingSystemOverride = null;
+  });
+
+  group('defaultTargetPlatform', () {
+    testWidgets('returns what ui_web says', (WidgetTester _) async {
+      // Set the OS reported by web_ui to anything that is not linux.
+      ui_web.browser.debugOperatingSystemOverride = ui_web.OperatingSystem.iOs;
+
+      expect(defaultTargetPlatform, TargetPlatform.iOS);
+    });
+
+    testWidgets('defaults `unknown` to android', (WidgetTester _) async {
+      ui_web.browser.debugOperatingSystemOverride = ui_web.OperatingSystem.unknown;
+
+      expect(defaultTargetPlatform, TargetPlatform.android);
+    });
+  });
+}