Fix incorrect platformchannel response when clipboard getData is not supported (#25743)

lib/web_ui/lib/src/engine/clipboard.dart

@@ -45,12 +45,29 @@ class ClipboardMessageHandler {
       final Map<String, dynamic> map = <String, dynamic>{'text': data};
       callback!(codec.encodeSuccessEnvelope(map));
     }).catchError((dynamic error) {
-      print('Could not get text from clipboard: $error');
-      callback!(codec.encodeErrorEnvelope(
-          code: 'paste_fail', message: 'Clipboard.getData failed'));
+      if (error is UnimplementedError) {
+        // Clipboard.getData not supported.
+        // Passing [null] to [callback] indicates that the platform message isn't
+        // implemented. Look at [MethodChannel.invokeMethod] to see how [null] is
+        // handled.
+        Future<void>.delayed(Duration.zero).then((_) {
+          if (callback != null) {
+            callback(null);
+          }
+        });
+        return;
+      }
+      _reportGetDataFailure(callback, codec, error);
     });
   }
 
+  void _reportGetDataFailure(ui.PlatformMessageResponseCallback? callback,
+      MethodCodec codec, dynamic error) {
+    print('Could not get text from clipboard: $error');
+    callback!(codec.encodeErrorEnvelope(
+        code: 'paste_fail', message: 'Clipboard.getData failed'));
+  }
+
   /// Methods used by tests.
   set pasteFromClipboardStrategy(PasteFromClipboardStrategy strategy) {
     _pasteFromClipboardStrategy = strategy;
@@ -185,6 +202,7 @@ class ExecCommandPasteStrategy implements PasteFromClipboardStrategy {
   @override
   Future<String> getData() {
     // TODO(nurhan): https://github.com/flutter/flutter/issues/48581
-    throw UnimplementedError('Paste is not implemented for this browser.');
+    return Future.error(
+        UnimplementedError('Paste is not implemented for this browser.'));
   }
 }

lib/web_ui/test/engine/platform_dispatcher_test.dart

@@ -3,6 +3,8 @@
 // found in the LICENSE file.
 
 import 'dart:async';
+import 'dart:html' as html;
+import 'dart:js_util' as js_util;
 import 'dart:typed_data';
 
 import 'package:ui/src/engine.dart';
@@ -55,5 +57,31 @@ void testMain() {
         true,
       );
     });
+
+    test('responds correctly to flutter/platform Clipboard.getData failure',
+        () async {
+      // Patch browser so that clipboard api is not available.
+      dynamic originalClipboard =
+          js_util.getProperty(html.window.navigator, 'clipboard');
+      js_util.setProperty(html.window.navigator, 'clipboard', null);
+      const MethodCodec codec = JSONMethodCodec();
+      final Completer<ByteData?> completer = Completer<ByteData?>();
+      ui.PlatformDispatcher.instance.sendPlatformMessage(
+        'flutter/platform',
+        codec.encodeMethodCall(MethodCall(
+          'Clipboard.getData',
+        )),
+        completer.complete,
+      );
+      final ByteData? response = await completer.future;
+      if (response != null) {
+        expect(
+              () => codec.decodeEnvelope(response),
+          throwsA(isA<PlatformException>()),
+        );
+      }
+      js_util.setProperty(
+          html.window.navigator, 'clipboard', originalClipboard);
+    });
   });
 }