Fold _FlutterTestRunnerImpl into FlutterTestRunner. (#161188)

Closes https://github.com/flutter/flutter/issues/160461.

This pattern is covered by `interface class { ... }` in modern Dart.

See <https://dart.dev/language/class-modifiers#interface>.

packages/flutter_tools/lib/src/test/runner.dart

@@ -26,78 +26,11 @@ import 'test_wrapper.dart';
 import 'watcher.dart';
 import 'web_test_compiler.dart';
 
-/// A class that abstracts launching the test process from the test runner.
-abstract class FlutterTestRunner {
-  const factory FlutterTestRunner() = _FlutterTestRunnerImpl;
+/// Launching the `flutter_tester` process from the test runner.
+interface class FlutterTestRunner {
+  const FlutterTestRunner();
 
   /// Runs tests using package:test and the Flutter engine.
-  Future<int> runTests(
-    TestWrapper testWrapper,
-    List<Uri> testFiles, {
-    required DebuggingOptions debuggingOptions,
-    List<String> names = const <String>[],
-    List<String> plainNames = const <String>[],
-    String? tags,
-    String? excludeTags,
-    bool enableVmService = false,
-    bool machine = false,
-    String? precompiledDillPath,
-    Map<String, String>? precompiledDillFiles,
-    bool updateGoldens = false,
-    TestWatcher? watcher,
-    required int? concurrency,
-    String? testAssetDirectory,
-    FlutterProject? flutterProject,
-    String? icudtlPath,
-    Directory? coverageDirectory,
-    bool web = false,
-    String? randomSeed,
-    String? reporter,
-    String? fileReporter,
-    String? timeout,
-    bool failFast = false,
-    bool runSkipped = false,
-    int? shardIndex,
-    int? totalShards,
-    Device? integrationTestDevice,
-    String? integrationTestUserIdentifier,
-    TestTimeRecorder? testTimeRecorder,
-    TestCompilerNativeAssetsBuilder? nativeAssetsBuilder,
-    required BuildInfo buildInfo,
-  });
-
-  /// Runs tests using the experimental strategy of spawning each test in a
-  /// separate lightweight Engine.
-  Future<int> runTestsBySpawningLightweightEngines(
-    List<Uri> testFiles, {
-    required DebuggingOptions debuggingOptions,
-    List<String> names = const <String>[],
-    List<String> plainNames = const <String>[],
-    String? tags,
-    String? excludeTags,
-    bool machine = false,
-    bool updateGoldens = false,
-    required int? concurrency,
-    String? testAssetDirectory,
-    FlutterProject? flutterProject,
-    String? icudtlPath,
-    String? randomSeed,
-    String? reporter,
-    String? fileReporter,
-    String? timeout,
-    bool failFast = false,
-    bool runSkipped = false,
-    int? shardIndex,
-    int? totalShards,
-    TestTimeRecorder? testTimeRecorder,
-    TestCompilerNativeAssetsBuilder? nativeAssetsBuilder,
-  });
-}
-
-class _FlutterTestRunnerImpl implements FlutterTestRunner {
-  const _FlutterTestRunnerImpl();
-
-  @override
   Future<int> runTests(
     TestWrapper testWrapper,
     List<Uri> testFiles, {
@@ -637,7 +570,8 @@ class SpawnPlugin extends PlatformPlugin {
     testTimeRecorder?.stop(TestTimePhases.Compile, testTimeRecorderStopwatch!);
   }
 
-  @override
+  /// Runs tests using the experimental strategy of spawning each test in a
+  /// separate lightweight Engine.
   Future<int> runTestsBySpawningLightweightEngines(
     List<Uri> testFiles, {
     required DebuggingOptions debuggingOptions,