Enable CupertinoContextMenu on Web (#52977)

packages/flutter/lib/src/cupertino/context_menu.dart

@@ -5,6 +5,7 @@
 import 'dart:math' as math;
 import 'dart:ui' as ui;
 import 'package:flutter/gestures.dart' show kMinFlingVelocity, kLongPressTimeout;
+import 'package:flutter/foundation.dart' show kIsWeb;
 import 'package:flutter/rendering.dart';
 import 'package:flutter/scheduler.dart';
 import 'package:flutter/services.dart';
@@ -511,7 +512,12 @@ class _DecoyChildState extends State<_DecoyChild> with TickerProviderStateMixin
       : _mask.value;
     return Positioned.fromRect(
       rect: _rect.value,
-      child: ShaderMask(
+      // TODO(justinmc): When ShaderMask is supported on web, remove this
+      // conditional and use ShaderMask everywhere.
+      // https://github.com/flutter/flutter/issues/52967.
+      child: kIsWeb
+          ? Container(key: _childGlobalKey, child: widget.child)
+          : ShaderMask(
             key: _childGlobalKey,
             shaderCallback: (Rect bounds) {
               return LinearGradient(

packages/flutter/test/cupertino/context_menu_test.dart

@@ -4,6 +4,7 @@
 
 import 'package:flutter_test/flutter_test.dart';
 import 'package:flutter/cupertino.dart';
+import 'package:flutter/foundation.dart' show kIsWeb;
 
 void main() {
   final TestWidgetsFlutterBinding binding =
@@ -103,6 +104,11 @@ void main() {
       Rect decoyChildRect = tester.getRect(_findDecoyChild(child));
       expect(childRect, equals(decoyChildRect));
 
+      // TODO(justinmc): When ShaderMask is supported on web, remove this
+      // conditional and just check for ShaderMask.
+      // https://github.com/flutter/flutter/issues/52967.
+      expect(find.byType(ShaderMask), kIsWeb ? findsNothing : findsOneWidget);
+
       // After a small delay, the _DecoyChild has begun to animate.
       await tester.pump(const Duration(milliseconds: 100));
       decoyChildRect = tester.getRect(_findDecoyChild(child));