Don't leak exceptions thrown by message channels (#74446)

packages/flutter_test/lib/src/binding.dart

@@ -104,10 +104,10 @@ class TestDefaultBinaryMessenger extends BinaryMessenger {
   Future<ByteData?>? send(String channel, ByteData? message) {
     final Future<ByteData?>? resultFuture = delegate.send(channel, message);
     if (resultFuture != null) {
-      // Removes the future itself from the [_pendingMessages] list when it
-      // completes.
       _pendingMessages.add(resultFuture);
-      resultFuture.whenComplete(() => _pendingMessages.remove(resultFuture));
+      resultFuture
+        .catchError((Object error) { /* errors are the responsibility of the caller */ })
+        .whenComplete(() => _pendingMessages.remove(resultFuture));
     }
     return resultFuture;
   }

packages/flutter_test/test/test_default_binary_messenger_test.dart

@@ -0,0 +1,45 @@
+// 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:typed_data';
+import 'dart:ui' as ui;
+
+import 'package:flutter/services.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+class TestDelegate extends BinaryMessenger {
+  @override
+  Future<ByteData?>? send(String channel, ByteData? message) async {
+    expect(channel, '');
+    expect(message, isNull);
+    throw 'Vic Fontaine';
+  }
+
+  // Rest of the API isn't needed for this test.
+  @override
+  Future<void> handlePlatformMessage(String channel, ByteData? data, ui.PlatformMessageResponseCallback? callback) => throw UnimplementedError();
+  @override
+  void setMessageHandler(String channel, MessageHandler? handler) => throw UnimplementedError();
+  @override
+  bool checkMessageHandler(String channel, MessageHandler? handler) => throw UnimplementedError();
+  @override
+  void setMockMessageHandler(String channel, MessageHandler? handler) => throw UnimplementedError();
+  @override
+  bool checkMockMessageHandler(String channel, MessageHandler? handler) => throw UnimplementedError();
+}
+
+void main() {
+  testWidgets('Caught exceptions are caught by the test framework', (WidgetTester tester) async {
+    final BinaryMessenger delegate = TestDelegate();
+    final Future<ByteData?>? future = delegate.send('', null);
+    expect(future, isNotNull);
+    await future!.catchError((Object error) { });
+    try {
+      await TestDefaultBinaryMessenger(delegate).send('', null);
+      expect(true, isFalse); // should not reach here
+    } catch (error) {
+      expect(error, 'Vic Fontaine');
+    }
+  });
+}