Handle Flutter.Error events and disable structure errors for noDebug mode (#100149)

packages/flutter_tools/lib/src/debug_adapters/flutter_adapter.dart

@@ -223,11 +223,29 @@ class FlutterDebugAdapter extends DartDebugAdapter<FlutterLaunchRequestArguments
           case 'Flutter.ServiceExtensionStateChanged':
             _sendServiceExtensionStateChanged(event.extensionData);
             break;
+          case 'Flutter.Error':
+            _handleFlutterErrorEvent(event.extensionData);
+            break;
         }
         break;
     }
   }
 
+  /// Sends OutputEvents to the client for a Flutter.Error event.
+  void _handleFlutterErrorEvent(vm.ExtensionData? data) {
+    final Map<String, dynamic>? errorData = data?.data;
+    if (errorData == null) {
+      return;
+    }
+
+    final String errorText = (errorData['renderedErrorText'] as String?)
+        ?? (errorData['description'] as String?)
+        // We should never not error text, but if we do at least send something
+        // so it's not just completely silent.
+        ?? 'Unknown error in Flutter.Error event';
+    sendOutput('stderr', '$errorText\n');
+  }
+
   /// Called by [launchRequest] to request that we actually start the app to be run/debugged.
   ///
   /// For debugging, this should start paused, connect to the VM Service, set
@@ -240,6 +258,12 @@ class FlutterDebugAdapter extends DartDebugAdapter<FlutterLaunchRequestArguments
       'run',
       '--machine',
       if (enableDebugger) '--start-paused',
+      // Structured errors are enabled by default, but since we don't connect
+      // the VM Service for noDebug, we need to disable them so that error text
+      // is sent to stderr. Otherwise the user will not see any exception text
+      // (because nobody is listening for Flutter.Error events).
+      if (!enableDebugger)
+        '--dart-define=flutter.inspector.structuredErrors=false',
     ];
 
     await _startProcess(

packages/flutter_tools/test/integration.shard/debug_adapter/flutter_adapter_test.dart

@@ -115,6 +115,40 @@ void main() {
       expect(output, contains('Exited (1)'));
     });
 
+    /// Helper that tests exception output in either debug or noDebug mode.
+    Future<void> testExceptionOutput({required bool noDebug}) async {
+        final BasicProjectThatThrows project = BasicProjectThatThrows();
+        await project.setUpIn(tempDir);
+
+        final List<OutputEventBody> outputEvents =
+            await dap.client.collectAllOutput(launch: () {
+          // Terminate the app after we see the exception because otherwise
+          // it will keep running and `collectAllOutput` won't end.
+          dap.client.output
+              .firstWhere((String output) => output.contains(endOfErrorOutputMarker))
+              .then((_) => dap.client.terminate());
+          return dap.client.launch(
+            noDebug: noDebug,
+            cwd: project.dir.path,
+            toolArgs: <String>['-d', 'flutter-tester'],
+          );
+        });
+
+        final String output = _uniqueOutputLines(outputEvents);
+        final List<String> outputLines = output.split('\n');
+        expect( outputLines, containsAllInOrder(<String>[
+            '══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════',
+            'The following _Exception was thrown building App(dirty):',
+            'Exception: c',
+            'The relevant error-causing widget was:',
+        ]));
+        expect(output, contains('App:${Uri.file(project.dir.path)}/lib/main.dart:24:12'));
+    }
+
+    testWithoutContext('correctly outputs exceptions in debug mode', () => testExceptionOutput(noDebug: false));
+
+    testWithoutContext('correctly outputs exceptions in noDebug mode', () => testExceptionOutput(noDebug: true));
+
     testWithoutContext('can hot reload', () async {
       final BasicProject project = BasicProject();
       await project.setUpIn(tempDir);

packages/flutter_tools/test/integration.shard/debug_adapter/test_support.dart

@@ -33,6 +33,9 @@ final bool useInProcessDap = Platform.environment['DAP_TEST_INTERNAL'] == 'true'
 /// Service traffic (wrapped in a custom 'dart.log' event).
 final bool verboseLogging = Platform.environment['DAP_TEST_VERBOSE'] == 'true';
 
+const String startOfErrorOutputMarker = '══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════';
+const String endOfErrorOutputMarker = '════════════════════════════════════════════════════════════════════════════════════════════════════';
+
 /// Expects the lines in [actual] to match the relevant matcher in [expected],
 /// ignoring differences in line endings and trailing whitespace.
 void expectLines(