[flutter_tools] Fix encoded stderr in "dart.log" from debug adapter to client (#155249)

To aid debugging, debug adapter (DAP) clients can ask the debug adapters
to send verbose logs back to the client (so they can capture them in a
client-side log along with other things happening on the client).
Included in this log is the `stderr` output of the `flutter run` process
spawned by the debug adapter.

This output was not decoded correctly for these logs, so showed up like:

```
[Flutter] [stderr] [91, 32, 32, 43, 49, 52, 32, 109, 115, 93, 32, 67, 111, 117, 108, 100, 32, 110, 111, 116, 32, 102, 105, 110, 100, 32, 97, 110, 32, 111, 112, 116, 105, 111, 110, 32, 110, 97, 109, 101, 100, 32, 34, 105, 110, 118, 97, 108, 105, 100, 34, 46, 10, 10, 91, 32, 32, 32, 32, 32, 32, 32, 32, 93, 32, 82, 117, 110, 32, 39, 102, 108, 117, 116, 116, 101, 114, 32, 45, 104, 39, 32, 40, 111, 114, 32, 39, 102, 108, 117, 116, 116, 101, 114, 32, 60, 99, 111, 109, 109, 97, 110, 100, 62, 32, 45, 104, 39, 41, 32, 102, 111, 114, 32, 97, 118, 97, 105, 108, 97, 98, 108, 101, 32, 102, 108, 117, 116, 116, 101, 114, 32, 99, 111, 109, 109, 97, 110, 100, 115, 32, 97, 110, 100, 32, 111, 112, 116, 105, 111, 110, 115, 46, 10]
```

This change decodes the output before processing.

Fixes https://github.com/Dart-Code/Dart-Code/issues/5268

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

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

@@ -603,9 +603,9 @@ class FlutterDebugAdapter extends FlutterBaseDebugAdapter with VmServiceInfoFile
   }
 
   @override
-  void handleStderr(List<int> data) {
+  void handleStderr(String data) {
     _logTraffic('<== [Flutter] [stderr] $data');
-    sendOutput('stderr', utf8.decode(data));
+    sendOutput('stderr', data);
   }
 
   /// Handles stdout from the `flutter run --machine` process, decoding the JSON and calling the appropriate handlers.

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

@@ -11,6 +11,7 @@ import '../base/file_system.dart';
 import '../base/io.dart';
 import '../base/platform.dart';
 import '../cache.dart';
+import '../convert.dart';
 import 'flutter_adapter_args.dart';
 import 'mixins.dart';
 
@@ -150,11 +151,11 @@ abstract class FlutterBaseDebugAdapter extends DartDebugAdapter<FlutterLaunchReq
     this.process = process;
 
     process.stdout.transform(ByteToLineTransformer()).listen(handleStdout);
-    process.stderr.listen(handleStderr);
+    process.stderr.transform(utf8.decoder).listen(handleStderr);
     unawaited(process.exitCode.then(handleExitCode));
   }
 
   void handleExitCode(int code);
-  void handleStderr(List<int> data);
+  void handleStderr(String data);
   void handleStdout(String data);
 }

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

@@ -105,9 +105,9 @@ class FlutterTestDebugAdapter extends FlutterBaseDebugAdapter with TestAdapter {
   }
 
   @override
-  void handleStderr(List<int> data) {
+  void handleStderr(String data) {
     logger?.call('stderr: $data');
-    sendOutput('stderr', utf8.decode(data));
+    sendOutput('stderr', data);
   }
 
   /// Handles stdout from the `flutter test --machine` process, decoding the JSON and calling the appropriate handlers.

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

@@ -74,7 +74,7 @@ void main() {
       );
     });
 
-    testWithoutContext('logs to client when sendLogsToClient=true', () async {
+    testWithoutContext('logs stdout to client when sendLogsToClient=true', () async {
       final BasicProject project = BasicProject();
       await project.setUpIn(tempDir);
 
@@ -107,6 +107,35 @@ void main() {
       );
     });
 
+    testWithoutContext('logs stderr to client when sendLogsToClient=true', () async {
+      final BasicProject project = BasicProject();
+      await project.setUpIn(tempDir);
+
+      // Capture all log events.
+      final Future<List<Event>> logEventsFuture = dap.client.events('dart.log').toList();
+
+      // Launch the app and wait for it to terminate (because of the error).
+      await Future.wait(<Future<void>>[
+        dap.client.event('terminated'),
+        dap.client.start(
+          launch: () => dap.client.launch(
+            cwd: project.dir.path,
+            noDebug: true,
+            toolArgs: <String>['--not-a-valid-flag'],
+            sendLogsToClient: true,
+          ),
+        ),
+      ], eagerError: true);
+
+      // Ensure logs contain the expected error message.
+      final List<Event> logEvents = await logEventsFuture;
+      final List<String> logMessages = logEvents.map((Event l) => (l.body! as Map<String, Object?>)['message']! as String).toList();
+      expect(
+        logMessages,
+        contains(startsWith('<== [Flutter] [stderr] Could not find an option named "not-a-valid-flag"')),
+      );
+    });
+
     testWithoutContext('can run and terminate a Flutter app in noDebug mode', () async {
       final BasicProject project = BasicProject();
       await project.setUpIn(tempDir);