Add a tag and assert some state in FlutterTestDriver tests. (#159099)

Towards https://github.com/flutter/flutter/issues/51421.

Asserts some state we would not like to be hit by in integration tests
using `Flutter*TestDriver`.

In addition, adds a tag that can be used to run all tests that currently
use this pattern:

```sh
flutter test --tags flutter-test-driver
```

This showcases processes leaking quite a bit:
<https://gist.github.com/matanlurey/190d9084d3a45bc5737b8406bc05b4cd>.

packages/flutter_tools/dart_test.yaml

@@ -16,3 +16,9 @@ tags:
   # the --test-randomize-ordering-seed for the suites that have this tag.
   no-shuffle:
     allow_test_randomization: false
+
+  # Tests that invoke `flutter build apk` for integration.
+  flutter-build-apk: {}
+
+  # Tests that invoke `flutter test ...` or `flutter run ...` for integration.
+  flutter-test-driver: {}

packages/flutter_tools/test/integration.shard/background_isolate_test.dart

@@ -2,6 +2,9 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+@Tags(<String>['flutter-test-driver'])
+library;
+
 import 'dart:async';
 
 import 'package:file/file.dart';

packages/flutter_tools/test/integration.shard/break_on_framework_exceptions_test.dart

@@ -2,6 +2,9 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+@Tags(<String>['flutter-test-driver'])
+library;
+
 import 'dart:async';
 
 import 'package:file/file.dart';

packages/flutter_tools/test/integration.shard/coverage_collection_test.dart

@@ -2,6 +2,9 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+@Tags(<String>['flutter-test-driver'])
+library;
+
 import 'package:file/file.dart';
 import 'package:file_testing/file_testing.dart';
 

packages/flutter_tools/test/integration.shard/debugger_stepping_test.dart

@@ -2,6 +2,9 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+@Tags(<String>['flutter-test-driver'])
+library;
+
 import 'package:file/file.dart';
 
 import '../src/common.dart';

packages/flutter_tools/test/integration.shard/deferred_components_test.dart

@@ -2,6 +2,9 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+@Tags(<String>['flutter-test-driver'])
+library;
+
 import 'package:archive/archive.dart';
 import 'package:file/file.dart';
 import 'package:file_testing/file_testing.dart';

packages/flutter_tools/test/integration.shard/flutter_attach_test.dart

@@ -2,6 +2,9 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+@Tags(<String>['flutter-test-driver'])
+library;
+
 import 'dart:convert';
 
 import 'package:file/file.dart';

packages/flutter_tools/test/integration.shard/flutter_gen_test.dart

@@ -2,6 +2,9 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+@Tags(<String>['flutter-test-driver'])
+library;
+
 import 'dart:convert';
 
 import 'package:file/file.dart';

packages/flutter_tools/test/integration.shard/flutter_run_test.dart

@@ -2,6 +2,9 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+@Tags(<String>['flutter-test-driver'])
+library;
+
 import 'package:file/file.dart';
 import 'package:flutter_tools/src/base/io.dart';
 import 'package:process/process.dart';

packages/flutter_tools/test/integration.shard/flutter_run_with_error_test.dart

@@ -2,6 +2,9 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+@Tags(<String>['flutter-test-driver'])
+library;
+
 import 'dart:async';
 
 import 'package:file/file.dart';

packages/flutter_tools/test/integration.shard/gen_l10n_test.dart

@@ -2,6 +2,9 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+@Tags(<String>['flutter-test-driver'])
+library;
+
 import 'dart:async';
 
 import 'package:file/file.dart';

packages/flutter_tools/test/integration.shard/hot_reload_errors_test.dart

@@ -2,6 +2,9 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+@Tags(<String>['flutter-test-driver'])
+library;
+
 import 'package:file/file.dart';
 
 import '../src/common.dart';

packages/flutter_tools/test/integration.shard/hot_reload_test.dart

@@ -2,6 +2,9 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+@Tags(<String>['flutter-test-driver'])
+library;
+
 import 'dart:async';
 
 import 'package:file/file.dart';

packages/flutter_tools/test/integration.shard/hot_reload_with_asset_test.dart

@@ -2,6 +2,9 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+@Tags(<String>['flutter-test-driver'])
+library;
+
 import 'dart:async';
 
 import 'package:file/file.dart';

packages/flutter_tools/test/integration.shard/lifetime_test.dart

@@ -2,6 +2,9 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+@Tags(<String>['flutter-test-driver'])
+library;
+
 import 'package:file/file.dart';
 
 import '../src/common.dart';

packages/flutter_tools/test/integration.shard/stateless_stateful_hot_reload_test.dart

@@ -2,6 +2,9 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+@Tags(<String>['flutter-test-driver'])
+library;
+
 import 'dart:async';
 
 import 'package:file/file.dart';

packages/flutter_tools/test/integration.shard/test_driver.dart

@@ -10,6 +10,7 @@ import 'package:file/file.dart';
 import 'package:flutter_tools/src/base/file_system.dart';
 import 'package:flutter_tools/src/base/io.dart';
 import 'package:flutter_tools/src/base/utils.dart';
+import 'package:meta/meta.dart';
 import 'package:process/process.dart';
 import 'package:vm_service/vm_service.dart';
 import 'package:vm_service/vm_service_io.dart';
@@ -35,7 +36,7 @@ const Duration defaultTimeout = Duration(seconds: 5);
 const Duration appStartTimeout = Duration(seconds: 120);
 const Duration quitTimeout = Duration(seconds: 10);
 
-abstract class FlutterTestDriver {
+abstract final class FlutterTestDriver {
   FlutterTestDriver(
     this._projectFolder, {
     String? logPrefix,
@@ -86,12 +87,17 @@ abstract class FlutterTestDriver {
     }
   }
 
+  @mustCallSuper
   Future<void> _setupProcess(
     List<String> arguments, {
     String? script,
     bool withDebugger = false,
     bool verbose = false,
   }) async {
+    if (_process != null && !_hasExited) {
+      throw StateError('Cannot start another process while the previous runs');
+    }
+
     if (withDebugger) {
       arguments.add('--start-paused');
     }
@@ -120,6 +126,9 @@ abstract class FlutterTestDriver {
     // via a getter for external uses.
     unawaited(_process!.exitCode.then((int code) {
       _debugPrint('Process exited ($code)');
+      // The timing of this signal is important to the implementation of the
+      // "quit" method, so only change how this is implemented by careful
+      // testing of tests that use FlutterTestDriver.
       _hasExited = true;
     }));
     transformToLines(_process!.stdout).listen(_stdout.add);
@@ -183,8 +192,24 @@ abstract class FlutterTestDriver {
     );
   }
 
-  Future<int> quit() => _killGracefully();
+  /// Quits the currently running process.
+  @nonVirtual
+  Future<void> quit() async {
+    final int result = await _killGracefully();
+    if (result != 0) {
+      _debugPrint('Expected process to terminate gracefully, got exit code $result.');
+    }
 
+    // The _hasExited signal could be on the microtask queue. Waiting for a
+    // Future(...) queues an event similar to Timer.run(Duration.zero), which
+    // guarantees the current queue has elapsed before moving on.
+    await Future<void>(() {});
+    if (!_hasExited) {
+      throw StateError('Process did not exit');
+    }
+  }
+
+  @nonVirtual
   Future<int> _killGracefully() async {
     if (_processPid == null) {
       return -1;
@@ -484,7 +509,7 @@ abstract class FlutterTestDriver {
   }
 }
 
-class FlutterRunTestDriver extends FlutterTestDriver {
+final class FlutterRunTestDriver extends FlutterTestDriver {
   FlutterRunTestDriver(
     super.projectFolder, {
     super.logPrefix,
@@ -770,7 +795,7 @@ class FlutterRunTestDriver extends FlutterTestDriver {
   final bool spawnDdsInstance;
 }
 
-class FlutterTestTestDriver extends FlutterTestDriver {
+final class FlutterTestTestDriver extends FlutterTestDriver {
   FlutterTestTestDriver(super.projectFolder, {super.logPrefix});
 
   Future<void> test({

packages/flutter_tools/test/integration.shard/timeline_test.dart

@@ -2,6 +2,9 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+@Tags(<String>['flutter-test-driver'])
+library;
+
 import 'dart:async';
 
 import 'package:file/file.dart';

packages/flutter_tools/test/integration.shard/vmservice_integration_test.dart

@@ -2,6 +2,9 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+@Tags(<String>['flutter-test-driver'])
+library;
+
 import 'package:file/file.dart';
 import 'package:flutter_tools/src/base/file_system.dart';
 import 'package:vm_service/vm_service.dart';

packages/flutter_tools/test/web.shard/debugger_stepping_web_test.dart

@@ -2,6 +2,9 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+@Tags(<String>['flutter-test-driver'])
+library;
+
 import 'package:file/file.dart';
 
 import '../integration.shard/test_data/stepping_project.dart';

packages/flutter_tools/test/web.shard/expression_evaluation_web_test.dart

@@ -2,6 +2,9 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+@Tags(<String>['flutter-test-driver'])
+library;
+
 import 'package:file/file.dart';
 import 'package:vm_service/vm_service.dart';
 

packages/flutter_tools/test/web.shard/hot_reload_web_test.dart

@@ -2,6 +2,9 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+@Tags(<String>['flutter-test-driver'])
+library;
+
 import 'dart:async';
 
 import 'package:file/file.dart';

packages/flutter_tools/test/web.shard/output_web_test.dart

@@ -2,6 +2,9 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+@Tags(<String>['flutter-test-driver'])
+library;
+
 import 'package:file/file.dart';
 import 'package:flutter_tools/src/base/file_system.dart';
 import 'package:vm_service/vm_service.dart';

packages/flutter_tools/test/web.shard/vm_service_web_test.dart

@@ -2,6 +2,9 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+@Tags(<String>['flutter-test-driver'])
+library;
+
 import 'dart:async';
 
 import 'package:file/file.dart';

packages/flutter_tools/test/web.shard/web_run_test.dart

@@ -2,6 +2,9 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+@Tags(<String>['flutter-test-driver'])
+library;
+
 import 'package:file/file.dart';
 import 'package:flutter_tools/src/base/file_system.dart';