Eagerly close browser tests.

lib/src/runner.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:async';
 import 'dart:io';
 
 import 'package:async/async.dart';
 
 import 'backend/group.dart';
@@ skipping 148 matching lines @@
       });
     }
 
     if (_debugOperation != null) await _debugOperation.cancel();
 
     if (_suiteSubscription != null) _suiteSubscription.cancel();
     _suiteSubscription = null;
 
     // Make sure we close the engine *before* the loader. Otherwise,
     // LoadSuites provided by the loader may get into bad states.
-    await _engine.close();
+    //
+    // We close the loader's browsers while we're closing the engine because
+    // browser tests don't store any state we care about and we want them to
+    // shut down without waiting for their tear-downs.
+    await Future.wait([
+      _loader.closeBrowsers(),
+      _engine.close()
+    ]);
     if (timer != null) timer.cancel();
     await _loader.close();
   });
 
   /// Return a stream of [LoadSuite]s in [_config.paths].
   ///
   /// Only tests that match [_config.pattern] will be included in the
   /// suites once they're loaded.
   Stream<LoadSuite> _loadSuites() {
     return mergeStreams(_config.paths.map((path) {
@@ skipping 119 matching lines @@
       await _debugOperation.valueOrCancellation();
     }).listen(null);
 
     var results = await Future.wait([
       _suiteSubscription.asFuture().then((_) => _engine.suiteSink.close()),
       _engine.run()
     ]);
     return results.last;
   }
 }