Support using lightweight Flutter Engines to run tests (#141726)

This PR implements the functionality described above and hides it behind
the `--experimental-faster-testing` flag of `flutter test`.

### The following are some performance measurements from test runs
conducted on GitHub Actions

run 1 logs:
https://github.com/derekxu16/flutter_test_ci/actions/runs/8008029772/attempts/1
run 2 logs:
https://github.com/derekxu16/flutter_test_ci/actions/runs/8008029772/attempts/2
run 3 logs:
https://github.com/derekxu16/flutter_test_ci/actions/runs/8008029772/attempts/3

**length of `flutter test --reporter=expanded test/animation
test/foundation` step**

run 1: 54s
run 2: 52s
run 3: 56s

average: 54s

**length of `flutter test --experimental-faster-testing
--reporter=expanded test/animation test/foundation` step**

run 1: 27s
run 2: 27s
run 3: 29s

average: 27.67s (~48.77% shorter than 54s)

**length of `flutter test --reporter=expanded test/animation
test/foundation test/gestures test/painting test/physics test/rendering
test/scheduler test/semantics test/services` step**

run 1: 260s
run 2: 270s
run 3: 305s

average: 278.33s


**length of `flutter test --experimental-faster-testing
--reporter=expanded test/animation test/foundation test/gestures
test/painting test/physics test/rendering test/scheduler test/semantics
test/services` step**

from a clean build (right after deleting the build folder):

run 1: 215s
run 2: 227s
run 3: 245s

average: 229s (~17.72% shorter than 278.33s)

Note that in reality, `test/material` was not passed to `flutter test`
in the trials below. All of the test files under `test/material` except
for `test/material/icons_test.dart` were listed out individually

**length of `flutter test --reporter=expanded test/material` step**

run 1: 408s
run 2: 421s
run 3: 451s

average: 426.67s

**length of `flutter test --experimental-faster-testing
--reporter=expanded test/material` step**

run 1: 382s
run 2: 373s
run 3: 400s

average: 385s (~9.77% shorter than 426.67s)

---------

Co-authored-by: Dan Field <[email protected]>

Author: derekxu16

packages/flutter_test/lib/src/binding.dart

@@ -195,6 +195,7 @@ abstract class TestWidgetsFlutterBinding extends BindingBase
   TestWidgetsFlutterBinding() : platformDispatcher = TestPlatformDispatcher(
     platformDispatcher: PlatformDispatcher.instance,
   ) {
+    platformDispatcher.defaultRouteNameTestValue = '/';
     debugPrint = debugPrintOverride;
     debugDisableShadows = disableShadows;
   }
@@ -246,6 +247,7 @@ abstract class TestWidgetsFlutterBinding extends BindingBase
   void reset() {
     _restorationManager?.dispose();
     _restorationManager = null;
+    platformDispatcher.defaultRouteNameTestValue = '/';
     resetGestureBinding();
     testTextInput.reset();
     if (registerTestTextInput) {

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

@@ -80,6 +80,11 @@ class TestCommand extends FlutterCommand with DeviceBasedDevelopmentArtifacts {
     addEnableImpellerFlag(verboseHelp: verboseHelp);
 
     argParser
+      ..addFlag('experimental-faster-testing',
+        negatable: false,
+        hide: !verboseHelp,
+        help: 'Run each test in a separate lightweight Flutter Engine to speed up testing.'
+      )
       ..addMultiOption('name',
         help: 'A regular expression matching substrings of the names of tests to run.',
         valueHelp: 'regexp',
@@ -350,6 +355,23 @@ class TestCommand extends FlutterCommand with DeviceBasedDevelopmentArtifacts {
       );
     }
 
+    bool experimentalFasterTesting = boolArg('experimental-faster-testing');
+    if (experimentalFasterTesting) {
+      if (_isIntegrationTest || isWeb) {
+        experimentalFasterTesting = false;
+        globals.printStatus(
+          '--experimental-faster-testing was parsed but will be ignored. This '
+          'option is not supported when running integration tests or web tests.',
+        );
+      } else if (_testFileUris.length == 1) {
+        experimentalFasterTesting = false;
+        globals.printStatus(
+          '--experimental-faster-testing was parsed but will be ignored. This '
+          'option should not be used when running a single test file.',
+        );
+      }
+    }
+
     final bool startPaused = boolArg('start-paused');
     if (startPaused && _testFileUris.length != 1) {
       throwToolExit(
@@ -402,6 +424,13 @@ class TestCommand extends FlutterCommand with DeviceBasedDevelopmentArtifacts {
       // Running with concurrency will result in deploying multiple test apps
       // on the connected device concurrently, which is not supported.
       jobs = 1;
+    } else if (experimentalFasterTesting) {
+      if (argResults!.wasParsed('concurrency')) {
+        globals.printStatus(
+          '-j/--concurrency was parsed but will be ignored. This option is not '
+          'compatible with --experimental-faster-testing.',
+        );
+      }
     }
 
     final int? shardIndex = int.tryParse(stringArg('shard-index') ?? '');
@@ -425,6 +454,25 @@ class TestCommand extends FlutterCommand with DeviceBasedDevelopmentArtifacts {
           'If you set --shard-index you need to also set --total-shards.');
     }
 
+    final bool enableVmService = boolArg('enable-vmservice');
+    if (experimentalFasterTesting && enableVmService) {
+      globals.printStatus(
+        '--enable-vmservice was parsed but will be ignored. This option is not '
+        'compatible with --experimental-faster-testing.',
+      );
+    }
+
+    final bool ipv6 = boolArg('ipv6');
+    if (experimentalFasterTesting && ipv6) {
+      // [ipv6] is set when the user desires for the test harness server to use
+      // IPv6, but a test harness server will not be started at all when
+      // [experimentalFasterTesting] is set.
+      globals.printStatus(
+        '--ipv6 was parsed but will be ignored. This option is not compatible '
+        'with --experimental-faster-testing.',
+      );
+    }
+
     final bool machine = boolArg('machine');
     CoverageCollector? collector;
     if (boolArg('coverage') || boolArg('merge-coverage') ||
@@ -487,35 +535,61 @@ class TestCommand extends FlutterCommand with DeviceBasedDevelopmentArtifacts {
     }
 
     final Stopwatch? testRunnerTimeRecorderStopwatch = testTimeRecorder?.start(TestTimePhases.TestRunner);
-    final int result = await testRunner.runTests(
-      testWrapper,
-      _testFileUris.toList(),
-      debuggingOptions: debuggingOptions,
-      names: names,
-      plainNames: plainNames,
-      tags: tags,
-      excludeTags: excludeTags,
-      watcher: watcher,
-      enableVmService: collector != null || startPaused || boolArg('enable-vmservice'),
-      ipv6: boolArg('ipv6'),
-      machine: machine,
-      updateGoldens: boolArg('update-goldens'),
-      concurrency: jobs,
-      testAssetDirectory: testAssetDirectory,
-      flutterProject: flutterProject,
-      web: isWeb,
-      randomSeed: stringArg('test-randomize-ordering-seed'),
-      reporter: stringArg('reporter'),
-      fileReporter: stringArg('file-reporter'),
-      timeout: stringArg('timeout'),
-      runSkipped: boolArg('run-skipped'),
-      shardIndex: shardIndex,
-      totalShards: totalShards,
-      integrationTestDevice: integrationTestDevice,
-      integrationTestUserIdentifier: stringArg(FlutterOptions.kDeviceUser),
-      testTimeRecorder: testTimeRecorder,
-      nativeAssetsBuilder: nativeAssetsBuilder,
-    );
+    final int result;
+    if (experimentalFasterTesting) {
+      assert(!isWeb && !_isIntegrationTest && _testFileUris.length > 1);
+      result = await testRunner.runTestsBySpawningLightweightEngines(
+        _testFileUris.toList(),
+        debuggingOptions: debuggingOptions,
+        names: names,
+        plainNames: plainNames,
+        tags: tags,
+        excludeTags: excludeTags,
+        machine: machine,
+        updateGoldens: boolArg('update-goldens'),
+        concurrency: jobs,
+        testAssetDirectory: testAssetDirectory,
+        flutterProject: flutterProject,
+        randomSeed: stringArg('test-randomize-ordering-seed'),
+        reporter: stringArg('reporter'),
+        fileReporter: stringArg('file-reporter'),
+        timeout: stringArg('timeout'),
+        runSkipped: boolArg('run-skipped'),
+        shardIndex: shardIndex,
+        totalShards: totalShards,
+        testTimeRecorder: testTimeRecorder,
+      );
+    } else {
+      result = await testRunner.runTests(
+        testWrapper,
+        _testFileUris.toList(),
+        debuggingOptions: debuggingOptions,
+        names: names,
+        plainNames: plainNames,
+        tags: tags,
+        excludeTags: excludeTags,
+        watcher: watcher,
+        enableVmService: collector != null || startPaused || enableVmService,
+        ipv6: ipv6,
+        machine: machine,
+        updateGoldens: boolArg('update-goldens'),
+        concurrency: jobs,
+        testAssetDirectory: testAssetDirectory,
+        flutterProject: flutterProject,
+        web: isWeb,
+        randomSeed: stringArg('test-randomize-ordering-seed'),
+        reporter: stringArg('reporter'),
+        fileReporter: stringArg('file-reporter'),
+        timeout: stringArg('timeout'),
+        runSkipped: boolArg('run-skipped'),
+        shardIndex: shardIndex,
+        totalShards: totalShards,
+        integrationTestDevice: integrationTestDevice,
+        integrationTestUserIdentifier: stringArg(FlutterOptions.kDeviceUser),
+        testTimeRecorder: testTimeRecorder,
+        nativeAssetsBuilder: nativeAssetsBuilder,
+      );
+    }
     testTimeRecorder?.stop(TestTimePhases.TestRunner, testRunnerTimeRecorderStopwatch!);
 
     if (collector != null) {