Fix web test flakiness; enable web golden tests (#52789)

* Fix web test flakiness; enable web golden tests

The fix is three-part:

- Only allow one test to load _and_ test at any point in time.
- Use a fresh Chrome instance for each test file.
- Increase Cirrus resources.

The first two changes only fix the "Unknown error loading" error, but not hanging tests. The resource increase also prevents hanging tests.

Other minor changes:

- Remove test batching (it's no longer necessary)
- Fix the Chrome class, which was using the wrong Completer.

.cirrus.yml

@@ -4,9 +4,9 @@
 web_shard_template: &WEB_SHARD_TEMPLATE
   only_if: "changesInclude('.cirrus.yml', 'dev/**', 'packages/flutter/**', 'packages/flutter_test/**', 'packages/flutter_tools/lib/src/test/**', 'packages/flutter_web_plugins/**', 'bin/internal/**') || $CIRRUS_PR == ''"
   environment:
-    # As of October 2019, the Web shards needed more than 6G of RAM.
+    # As of March 2020, the Web shards needed 16G of RAM and 4 CPUs to run all framework tests with goldens without flaking.
-    CPU: 2
+    CPU: 4
-    MEMORY: 8G
+    MEMORY: 16G
     CHROME_NO_SANDBOX: true
     GOLD_SERVICE_ACCOUNT: ENCRYPTED[3afeea5ac7201151c3d0dc9648862f0462b5e4f55dc600ca8b692319622f7c3eda3d577b1b16cc2ef0311b7314c1c095]
   script:

dev/bots/test.dart

@@ -53,11 +53,6 @@ const int kDeviceLabShardCount = 4;
 /// The last shard also runs the Web plugin tests.
 const int kWebShardCount = 8;
 
-/// Maximum number of Web tests to run in a single `flutter test`. We found that
-/// large batches can get flaky, possibly because we reuse a single instance of
-/// the browser, and after many tests the browser's state gets corrupted.
-const int kWebBatchSize = 20;
-
 /// Tests that we don't run on Web for various reasons.
 //
 // TODO(yjbanov): we're getting rid of this blacklist as part of https://github.com/flutter/flutter/projects/60
@@ -685,11 +680,6 @@ Future<void> _runWebDebugTest(String target) async {
 }
 
 Future<void> _runFlutterWebTest(String workingDirectory, List<String> tests) async {
-  final List<String> batch = <String>[];
-  for (int i = 0; i < tests.length; i += 1) {
-    final String testFilePath = tests[i];
-    batch.add(testFilePath);
-    if (batch.length == kWebBatchSize || i == tests.length - 1) {
   await runCommand(
     flutter,
     <String>[
@@ -699,7 +689,7 @@ Future<void> _runFlutterWebTest(String workingDirectory, List<String> tests) asy
       '-v',
       '--platform=chrome',
       ...?flutterTestArgs,
-          ...batch,
+      ...tests,
     ],
     workingDirectory: workingDirectory,
     environment: <String, String>{
@@ -707,9 +697,6 @@ Future<void> _runFlutterWebTest(String workingDirectory, List<String> tests) asy
       'FLUTTER_LOW_RESOURCE_MODE': 'true',
     },
   );
-      batch.clear();
-    }
-  }
 }
 
 Future<void> _pubRunTest(String workingDirectory, {

packages/flutter/test/material/bottom_app_bar_theme_test.dart

@@ -82,7 +82,7 @@ void main() {
       find.byKey(_painterKey),
       matchesGoldenFile('bottom_app_bar_theme.custom_shape.png'),
     );
-  }, skip: isBrowser);
+  });
 
   testWidgets('BAB theme does not affect defaults', (WidgetTester tester) async {
     await tester.pumpWidget(const MaterialApp(

packages/flutter/test/material/bottom_navigation_bar_test.dart

@@ -913,7 +913,7 @@ void main() {
     await tester.tap(find.text('Alarm'));
     await tester.pump(const Duration(seconds: 1));
     expect(Theme.of(tester.element(find.text('Alarm'))).brightness, equals(Brightness.dark));
-  }, skip: isBrowser);
+  });
 
   testWidgets('BottomNavigationBar iconSize test', (WidgetTester tester) async {
     double builderIconSize;
@@ -1023,7 +1023,7 @@ void main() {
 
     final RenderBox box = tester.renderObject(find.byType(BottomNavigationBar));
     expect(box.size.height, equals(66.0));
-  }, skip: isBrowser);
+  });
 
   testWidgets('BottomNavigationBar limits width of tiles with long titles', (WidgetTester tester) async {
     final Text longTextA = Text(''.padLeft(100, 'A'));
@@ -1055,7 +1055,7 @@ void main() {
     expect(itemBoxA.size, equals(const Size(400.0, 14.0)));
     final RenderBox itemBoxB = tester.renderObject(find.text(longTextB.data));
     expect(itemBoxB.size, equals(const Size(400.0, 14.0)));
-  }, skip: isBrowser);
+  });
 
   testWidgets('BottomNavigationBar paints circles', (WidgetTester tester) async {
     await tester.pumpWidget(
@@ -1125,7 +1125,7 @@ void main() {
           ..translate(x: 400.0)
           ..circle(x: 200.0),
     );
-  }, skip: isBrowser);
+  });
 
   testWidgets('BottomNavigationBar inactiveIcon shown', (WidgetTester tester) async {
     const Key filled = Key('filled');
@@ -1452,7 +1452,7 @@ void main() {
           find.byType(BottomNavigationBar),
           matchesGoldenFile('bottom_navigation_bar.shifting_transition.${pump - 1}.png'),
         );
-      }, skip: isBrowser); // TODO(yjbanov): web does not support golden tests yet: https://github.com/flutter/flutter/issues/40297
+      });
     }
   });
 

packages/flutter/test/material/dialog_theme_test.dart

@@ -132,7 +132,7 @@ void main() {
       find.byKey(_painterKey),
       matchesGoldenFile('dialog_theme.dialog_with_custom_border.png'),
     );
-  }, skip: isBrowser);
+  });
 
   testWidgets('Custom Title Text Style - Constructor Param', (WidgetTester tester) async {
     const String titleText = 'Title';

packages/flutter/test/material/dropdown_test.dart

@@ -189,7 +189,7 @@ void main() {
       find.ancestor(of: buttonFinder, matching: find.byType(RepaintBoundary)).first,
       matchesGoldenFile('dropdown_test.default.png'),
     );
-  }, skip: isBrowser);
+  });
 
   testWidgets('Expanded dropdown golden', (WidgetTester tester) async {
     final Key buttonKey = UniqueKey();
@@ -201,7 +201,7 @@ void main() {
       find.ancestor(of: buttonFinder, matching: find.byType(RepaintBoundary)).first,
       matchesGoldenFile('dropdown_test.expanded.png'),
     );
-  }, skip: isBrowser);
+  });
 
   testWidgets('Dropdown button control test', (WidgetTester tester) async {
     String value = 'one';

packages/flutter/test/material/flexible_space_bar_stretch_mode_test.dart

@@ -85,7 +85,7 @@ void main() {
       find.byType(FlexibleSpaceBar),
       matchesGoldenFile('flexible_space_bar_stretch_mode.blur_background.png'),
     );
-  }, skip: isBrowser);
+  });
 
   testWidgets('FlexibleSpaceBar stretch mode fadeTitle', (WidgetTester tester) async {
     await tester.pumpWidget(

packages/flutter/test/material/radio_test.dart

@@ -282,7 +282,7 @@ void main() {
       find.byKey(painterKey),
       matchesGoldenFile('radio.ink_ripple.png'),
     );
-  }, skip: isBrowser);
+  });
 
   testWidgets('Radio is focusable and has correct focus color', (WidgetTester tester) async {
     final FocusNode focusNode = FocusNode(debugLabel: 'Radio');

packages/flutter/test/material/tab_bar_theme_test.dart

@@ -269,7 +269,7 @@ void main() {
       find.byKey(_painterKey),
       matchesGoldenFile('tab_bar_theme.tab_indicator_size_tab.png'),
     );
-  }, skip: isBrowser);
+  });
 
   testWidgets('Tab bar theme overrides tab indicator size (label)', (WidgetTester tester) async {
     const TabBarTheme tabBarTheme = TabBarTheme(indicatorSize: TabBarIndicatorSize.label);
@@ -280,7 +280,7 @@ void main() {
       find.byKey(_painterKey),
       matchesGoldenFile('tab_bar_theme.tab_indicator_size_label.png'),
     );
-  }, skip: isBrowser);
+  });
 
   testWidgets('Tab bar theme - custom tab indicator', (WidgetTester tester) async {
     final TabBarTheme tabBarTheme = TabBarTheme(
@@ -296,7 +296,7 @@ void main() {
       find.byKey(_painterKey),
       matchesGoldenFile('tab_bar_theme.custom_tab_indicator.png'),
     );
-  }, skip: isBrowser);
+  });
 
   testWidgets('Tab bar theme - beveled rect indicator', (WidgetTester tester) async {
     final TabBarTheme tabBarTheme = TabBarTheme(
@@ -312,5 +312,5 @@ void main() {
       find.byKey(_painterKey),
       matchesGoldenFile('tab_bar_theme.beveled_rect_indicator.png'),
     );
-  }, skip: isBrowser);
+  });
 }

packages/flutter/test/rendering/localized_fonts_test.dart

@@ -52,7 +52,6 @@ void main() {
         matchesGoldenFile('localized_fonts.rich_text.styled_text_span.png'),
       );
     },
-    skip: isBrowser, // TODO(yjbanov): implement goldens on the Web: https://github.com/flutter/flutter/issues/40297
   );
 
   testWidgets(

packages/flutter/test/widgets/backdrop_filter_test.dart

@@ -45,5 +45,5 @@ void main() {
       find.byType(RepaintBoundary).first,
       matchesGoldenFile('backdrop_filter_test.cull_rect.png'),
     );
-  }, skip: isBrowser);
+  });
 }

packages/flutter/test/widgets/clip_test.dart

@@ -383,7 +383,7 @@ void main() {
       find.byType(RepaintBoundary).first,
       matchesGoldenFile('clip.ClipRect.png'),
     );
-  }, skip: isBrowser);
+  });
 
   testWidgets('ClipRect save, overlay, and antialiasing', (WidgetTester tester) async {
     await tester.pumpWidget(
@@ -423,7 +423,7 @@ void main() {
       find.byType(RepaintBoundary).first,
       matchesGoldenFile('clip.ClipRectOverlay.png'),
     );
-  }, skip: isBrowser);
+  });
 
   testWidgets('ClipRRect painting', (WidgetTester tester) async {
     await tester.pumpWidget(
@@ -472,7 +472,7 @@ void main() {
       find.byType(RepaintBoundary).first,
       matchesGoldenFile('clip.ClipRRect.png'),
     );
-  }, skip: isBrowser);
+  });
 
   testWidgets('ClipOval painting', (WidgetTester tester) async {
     await tester.pumpWidget(
@@ -515,7 +515,7 @@ void main() {
       find.byType(RepaintBoundary).first,
       matchesGoldenFile('clip.ClipOval.png'),
     );
-  }, skip: isBrowser);
+  });
 
   testWidgets('ClipPath painting', (WidgetTester tester) async {
     await tester.pumpWidget(
@@ -563,7 +563,7 @@ void main() {
       find.byType(RepaintBoundary).first,
       matchesGoldenFile('clip.ClipPath.png'),
     );
-  }, skip: isBrowser);
+  });
 
   Center genPhysicalModel(Clip clipBehavior) {
     return Center(
@@ -608,7 +608,7 @@ void main() {
       find.byType(RepaintBoundary).first,
       matchesGoldenFile('clip.PhysicalModel.antiAlias.png'),
     );
-  }, skip: isBrowser);
+  });
 
   testWidgets('PhysicalModel painting with Clip.hardEdge', (WidgetTester tester) async {
     await tester.pumpWidget(genPhysicalModel(Clip.hardEdge));
@@ -616,7 +616,7 @@ void main() {
       find.byType(RepaintBoundary).first,
       matchesGoldenFile('clip.PhysicalModel.hardEdge.png'),
     );
-  }, skip: isBrowser);
+  });
 
   // There will be bleeding edges on the rect edges, but there shouldn't be any bleeding edges on the
   // round corners.
@@ -626,7 +626,7 @@ void main() {
       find.byType(RepaintBoundary).first,
       matchesGoldenFile('clip.PhysicalModel.antiAliasWithSaveLayer.png'),
     );
-  }, skip: isBrowser);
+  });
 
   testWidgets('Default PhysicalModel painting', (WidgetTester tester) async {
     await tester.pumpWidget(
@@ -668,7 +668,7 @@ void main() {
       find.byType(RepaintBoundary).first,
       matchesGoldenFile('clip.PhysicalModel.default.png'),
     );
-  }, skip: isBrowser);
+  });
 
   Center genPhysicalShape(Clip clipBehavior) {
     return Center(
@@ -717,7 +717,7 @@ void main() {
       find.byType(RepaintBoundary).first,
       matchesGoldenFile('clip.PhysicalShape.antiAlias.png'),
     );
-  }, skip: isBrowser);
+  });
 
   testWidgets('PhysicalShape painting with Clip.hardEdge', (WidgetTester tester) async {
     await tester.pumpWidget(genPhysicalShape(Clip.hardEdge));
@@ -725,7 +725,7 @@ void main() {
       find.byType(RepaintBoundary).first,
       matchesGoldenFile('clip.PhysicalShape.hardEdge.png'),
     );
-  }, skip: isBrowser);
+  });
 
   testWidgets('PhysicalShape painting with Clip.antiAliasWithSaveLayer', (WidgetTester tester) async {
     await tester.pumpWidget(genPhysicalShape(Clip.antiAliasWithSaveLayer));
@@ -733,7 +733,7 @@ void main() {
       find.byType(RepaintBoundary).first,
       matchesGoldenFile('clip.PhysicalShape.antiAliasWithSaveLayer.png'),
     );
-  }, skip: isBrowser);
+  });
 
   testWidgets('PhysicalShape painting', (WidgetTester tester) async {
     await tester.pumpWidget(
@@ -779,7 +779,7 @@ void main() {
       find.byType(RepaintBoundary).first,
       matchesGoldenFile('clip.PhysicalShape.default.png'),
     );
-  }, skip: isBrowser);
+  });
 
   testWidgets('ClipPath.shape', (WidgetTester tester) async {
     final List<String> logs = <String>[];

packages/flutter/test/widgets/invert_colors_test.dart

@@ -22,7 +22,7 @@ void main() {
       find.byType(RepaintBoundary),
       matchesGoldenFile('invert_colors_test.0.png'),
     );
-  }, skip: isBrowser);
+  });
 
   testWidgets('InvertColors and ColorFilter',  (WidgetTester tester) async {
     await tester.pumpWidget(const RepaintBoundary(
@@ -40,7 +40,7 @@ void main() {
       find.byType(RepaintBoundary),
       matchesGoldenFile('invert_colors_test.1.png'),
     );
-  }, skip: isBrowser);
+  });
 }
 
 // Draws a rectangle sized by the parent widget with [color], [colorFilter],

packages/flutter/test/widgets/list_wheel_scroll_view_test.dart

@@ -594,7 +594,7 @@ void main() {
         find.byKey(const Key('list_wheel_scroll_view')),
         matchesGoldenFile('list_wheel_scroll_view.center_child.magnified.png'),
       );
-    }, skip: isBrowser);
+    });
 
     testWidgets('Default middle transform', (WidgetTester tester) async {
       await tester.pumpWidget(
@@ -648,7 +648,7 @@ void main() {
         find.byKey(const Key('list_wheel_scroll_view')),
         matchesGoldenFile('list_wheel_scroll_view.curved_wheel.left.png'),
       );
-    }, skip: isBrowser);
+    });
 
     testWidgets('Scrolling, diameterRatio, perspective all changes matrix', (WidgetTester tester) async {
       final ScrollController controller = ScrollController(initialScrollOffset: 200.0);

packages/flutter/test/widgets/opacity_test.dart

@@ -180,7 +180,7 @@ void main() {
       find.byType(RepaintBoundary).first,
       matchesGoldenFile('opacity_test.offset.png'),
     );
-  }, skip: isBrowser);
+  });
 
   testWidgets('empty opacity does not crash', (WidgetTester tester) async {
     await tester.pumpWidget(

packages/flutter/test/widgets/physical_model_test.dart

@@ -112,7 +112,7 @@ void main() {
       find.byKey(key),
       matchesGoldenFile('physical_model_overflow.png'),
     );
-  }, skip: isBrowser);
+  });
 
   group('PhysicalModelLayer checks elevation', () {
     Future<void> _testStackChildren(

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

@@ -256,10 +256,8 @@ class FlutterWebPlatform extends PlatformPlugin {
       Uint8List bytes;
 
       try {
-        final Runtime browser = Runtime.chrome;
+        final ChromeTab chromeTab = await _browserManager._browser.chromeConnection.getTab((ChromeTab tab) {
-        final BrowserManager browserManager = await _browserManagerFor(browser);
+          return tab.url.contains(_browserManager._browser.url);
-        final ChromeTab chromeTab = await browserManager._browser.chromeConnection.getTab((ChromeTab tab) {
-          return tab.url.contains(browserManager._browser.url);
         });
         final WipConnection connection = await chromeTab.connect();
         final WipResponse response = await connection.sendCommand('Page.captureScreenshot', <String, Object>{
@@ -303,10 +301,7 @@ class FlutterWebPlatform extends PlatformPlugin {
 
   bool get _closed => _closeMemo.hasRun;
 
-  // A map from browser identifiers to futures that will complete to the
+  BrowserManager _browserManager;
-  // [BrowserManager]s for those browsers, or `null` if they failed to load.
-  final Map<Runtime, Future<BrowserManager>> _browserManagers =
-      <Runtime, Future<BrowserManager>>{};
 
   // A handler that serves wrapper files used to bootstrap tests.
   shelf.Response _wrapperHandler(shelf.Request request) {
@@ -330,6 +325,11 @@ class FlutterWebPlatform extends PlatformPlugin {
     return shelf.Response.notFound('Not found.');
   }
 
+  /// Allows only one test suite (typically one test file) to be loaded and run
+  /// at any given point in time. Loading more than one file at a time is known
+  /// to lead to flaky tests.
+  final Pool _suiteLock = Pool(1);
+
   @override
   Future<RunnerSuite> load(
     String path,
@@ -340,17 +340,28 @@ class FlutterWebPlatform extends PlatformPlugin {
     if (_closed) {
       return null;
     }
+    final PoolResource lockResource = await _suiteLock.request();
+
     final Runtime browser = platform.runtime;
-    final BrowserManager browserManager = await _browserManagerFor(browser);
+    try {
-    if (_closed || browserManager == null) {
+      _browserManager = await _launchBrowser(browser);
+    } on Error catch (_) {
+      await _suiteLock.close();
+      rethrow;
+    }
+
+    if (_closed) {
       return null;
     }
 
     final Uri suiteUrl = url.resolveUri(globals.fs.path.toUri(globals.fs.path.withoutExtension(
             globals.fs.path.relative(path, from: globals.fs.path.join(_root, 'test'))) +
         '.html'));
-    final RunnerSuite suite = await browserManager
+    final RunnerSuite suite = await _browserManager.load(path, suiteUrl, suiteConfig, message, onDone: () async {
-        .load(path, suiteUrl, suiteConfig, message);
+      await _browserManager.close();
+      _browserManager = null;
+      lockResource.release();
+    });
     if (_closed) {
       return null;
     }
@@ -364,11 +375,11 @@ class FlutterWebPlatform extends PlatformPlugin {
   /// Returns the [BrowserManager] for [runtime], which should be a browser.
   ///
   /// If no browser manager is running yet, starts one.
-  Future<BrowserManager> _browserManagerFor(Runtime browser) {
+  Future<BrowserManager> _launchBrowser(Runtime browser) {
-    final Future<BrowserManager> managerFuture = _browserManagers[browser];
+    if (_browserManager != null) {
-    if (managerFuture != null) {
+      throw StateError('Another browser is currently running.');
-      return managerFuture;
     }
+
     final Completer<WebSocketChannel> completer =
         Completer<WebSocketChannel>.sync();
     final String path =
@@ -383,48 +394,29 @@ class FlutterWebPlatform extends PlatformPlugin {
 
     globals.printTrace('Serving tests at $hostUrl');
 
-    final Future<BrowserManager> future = BrowserManager.start(
+    return BrowserManager.start(
       browser,
       hostUrl,
       completer.future,
       headless: !_config.pauseAfterLoad,
     );
-
-    // Store null values for browsers that error out so we know not to load them
-    // again.
-    _browserManagers[browser] = future.catchError((dynamic _) => null);
-
-    return future;
   }
 
   @override
-  Future<void> closeEphemeral() {
+  Future<void> closeEphemeral() async {
-    final List<Future<BrowserManager>> managers =
+    if (_browserManager != null) {
-        _browserManagers.values.toList();
+      await _browserManager.close();
-    _browserManagers.clear();
-    return Future.wait(managers.map((Future<BrowserManager> manager) async {
-      final BrowserManager result = await manager;
-      if (result == null) {
-        return;
     }
-      await result.close();
-    }));
   }
 
   @override
   Future<void> close() => _closeMemo.runOnce(() async {
-    final List<Future<dynamic>> futures = _browserManagers.values
+    await Future.wait<void>(<Future<dynamic>>[
-      .map<Future<dynamic>>((Future<BrowserManager> future) async {
+      if (_browserManager != null)
-        final BrowserManager result = await future;
+        _browserManager.close(),
-        if (result == null) {
+      _server.close(),
-          return;
+      _testGoldenComparator.close(),
-        }
+    ]);
-        await result.close();
-      })
-      .toList();
-    futures.add(_server.close());
-    futures.add(_testGoldenComparator.close());
-    await Future.wait<void>(futures);
   });
 }
 
@@ -578,21 +570,6 @@ class BrowserManager {
   /// This is connected to a page running `static/host.dart`.
   MultiChannel<dynamic> _channel;
 
-  /// A pool that ensures that limits the number of initial connections the
-  /// manager will wait for at once.
-  ///
-  /// This isn't the *total* number of connections; any number of iframes may be
-  /// loaded in the same browser. However, the browser can only load so many at
-  /// once, and we want a timeout in case they fail so we only wait for so many
-  /// at once.
-  // The number 1 is chosen to disallow multiple iframes in the same browser. This
-  // is because in some environments, such as Cirrus CI, tests end up stuck and
-  // time out eventually. The exact reason for timeouts is unknown, but the
-  // hypothesis is that we were the first ones to attempt to run DDK-compiled
-  // tests concurrently in the browser. DDK is known to produce an order of
-  // magnitude bigger and somewhat slower code, which may overload the browser.
-  final Pool _pool = Pool(1);
-
   /// The ID of the next suite to be loaded.
   ///
   /// This is used to ensure that the suites can be referred to consistently
@@ -654,8 +631,8 @@ class BrowserManager {
 
     final Completer<BrowserManager> completer = Completer<BrowserManager>();
 
-    unawaited(chrome.onExit.then((void _) {
+    unawaited(chrome.onExit.then((int browserExitCode) {
-      throwToolExit('${runtime.name} exited before connecting.');
+      throwToolExit('${runtime.name} exited with code $browserExitCode before connecting.');
     }).catchError((dynamic error, StackTrace stackTrace) {
       if (completer.isCompleted) {
         return;
@@ -700,7 +677,9 @@ class BrowserManager {
     String path,
     Uri url,
     SuiteConfiguration suiteConfig,
-    Object message,
+    Object message, {
+      Future<void> Function() onDone,
+    }
   ) async {
     url = url.replace(fragment: Uri.encodeFull(jsonEncode(<String, Object>{
       'metadata': suiteConfig.metadata.serialize(),
@@ -726,10 +705,10 @@ class BrowserManager {
       StreamTransformer<dynamic, dynamic>.fromHandlers(handleDone: (EventSink<dynamic> sink) {
         closeIframe();
         sink.close();
+        onDone();
       }),
     );
 
-    return await _pool.withResource<RunnerSuite>(() async {
     _channel.sink.add(<String, Object>{
       'command': 'loadSuite',
       'url': url.toString(),
@@ -748,7 +727,6 @@ class BrowserManager {
       closeIframe();
       rethrow;
     }
-    });
   }
 
   /// An implementation of [Environment.displayPause].

packages/flutter_tools/lib/src/web/chrome.dart

@@ -116,6 +116,10 @@ class ChromeLauncher {
   ///
   /// `skipCheck` does not attempt to make a devtools connection before returning.
   Future<Chrome> launch(String url, { bool headless = false, int debugPort, bool skipCheck = false, Directory dataDir }) async {
+    if (_currentCompleter.isCompleted) {
+      throwToolExit('Only one instance of chrome can be started.');
+    }
+
     // This is a JSON file which contains configuration from the
     // browser session, such as window position. It is located
     // under the Chrome data-dir folder.
@@ -203,9 +207,6 @@ class ChromeLauncher {
   }
 
   static Future<Chrome> _connect(Chrome chrome, bool skipCheck) async {
-    if (_currentCompleter.isCompleted) {
-      throwToolExit('Only one instance of chrome can be started.');
-    }
     // The connection is lazy. Try a simple call to make sure the provided
     // connection is valid.
     if (!skipCheck) {
@@ -262,13 +263,11 @@ class Chrome {
   final ChromeConnection chromeConnection;
   final Uri remoteDebuggerUri;
 
-  static Completer<Chrome> _currentCompleter = Completer<Chrome>();
+  Future<int> get onExit => _process.exitCode;
-
-  Future<void> get onExit => _currentCompleter.future;
 
   Future<void> close() async {
-    if (_currentCompleter.isCompleted) {
+    if (ChromeLauncher.hasChromeInstance) {
-      _currentCompleter = Completer<Chrome>();
+      ChromeLauncher._currentCompleter = Completer<Chrome>();
     }
     chromeConnection.close();
     _process?.kill();

packages/flutter_tools/test/general.shard/web/chrome_test.dart

@@ -5,6 +5,7 @@
 import 'dart:async';
 
 import 'package:file/memory.dart';
+import 'package:flutter_tools/src/base/common.dart';
 import 'package:flutter_tools/src/base/file_system.dart';
 import 'package:flutter_tools/src/base/logger.dart';
 import 'package:flutter_tools/src/base/os.dart';
@@ -62,28 +63,31 @@ void main() {
   });
 
   test('can launch chrome and connect to the devtools', () async {
-    processManager.addCommand(const FakeCommand(
+    await testLaunchChrome('/.tmp_rand0/flutter_tool.rand0', processManager, chromeLauncher);
-      command: <String>[
+  });
-        'example_chrome',
-        '--user-data-dir=/.tmp_rand0/flutter_tool.rand0',
-        '--remote-debugging-port=1234',
-        ..._kChromeArgs,
-        'example_url',
-      ],
-      stderr: kDevtoolsStderr,
-    ));
 
-    await chromeLauncher.launch(
+  test('cannot have two concurrent instances of chrome', () async {
-      'example_url',
+    await testLaunchChrome('/.tmp_rand0/flutter_tool.rand0', processManager, chromeLauncher);
-      skipCheck: true,
+    bool pass = false;
-    );
+    try {
+      await testLaunchChrome('/.tmp_rand0/flutter_tool.rand1', processManager, chromeLauncher);
+    } on ToolExit catch (_) {
+      pass = true;
+    }
+    expect(pass, isTrue);
+  });
+
+  test('can launch new chrome after stopping a previous chrome', () async {
+    final Chrome  chrome = await testLaunchChrome('/.tmp_rand0/flutter_tool.rand0', processManager, chromeLauncher);
+    await chrome.close();
+    await testLaunchChrome('/.tmp_rand0/flutter_tool.rand1', processManager, chromeLauncher);
   });
 
   test('can launch chrome with a custom debug port', () async {
     processManager.addCommand(const FakeCommand(
       command: <String>[
         'example_chrome',
-        '--user-data-dir=/.tmp_rand0/flutter_tool.rand0',
+        '--user-data-dir=/.tmp_rand1/flutter_tool.rand1',
         '--remote-debugging-port=10000',
         ..._kChromeArgs,
         'example_url',
@@ -102,7 +106,7 @@ void main() {
     processManager.addCommand(const FakeCommand(
       command: <String>[
         'example_chrome',
-        '--user-data-dir=/.tmp_rand0/flutter_tool.rand0',
+        '--user-data-dir=/.tmp_rand1/flutter_tool.rand1',
         '--remote-debugging-port=1234',
         ..._kChromeArgs,
         '--headless',
@@ -133,7 +137,7 @@ void main() {
 
     processManager.addCommand(FakeCommand(command: const <String>[
       'example_chrome',
-      '--user-data-dir=/.tmp_rand0/flutter_tool.rand0',
+      '--user-data-dir=/.tmp_rand1/flutter_tool.rand1',
       '--remote-debugging-port=1234',
       ..._kChromeArgs,
       'example_url',
@@ -146,7 +150,7 @@ void main() {
     );
 
     final File tempFile = fileSystem
-      .directory('.tmp_rand0/flutter_tool.rand0')
+      .directory('.tmp_rand1/flutter_tool.rand1')
       .childDirectory('Default')
       .childFile('preferences');
 
@@ -163,3 +167,21 @@ void main() {
 }
 
 class MockOperatingSystemUtils extends Mock implements OperatingSystemUtils {}
+
+Future<Chrome> testLaunchChrome(String userDataDir, FakeProcessManager processManager, ChromeLauncher chromeLauncher) {
+  processManager.addCommand(FakeCommand(
+    command: <String>[
+      'example_chrome',
+      '--user-data-dir=$userDataDir',
+      '--remote-debugging-port=1234',
+      ..._kChromeArgs,
+      'example_url',
+    ],
+    stderr: kDevtoolsStderr,
+  ));
+
+  return chromeLauncher.launch(
+    'example_url',
+    skipCheck: true,
+  );
+}