Add the ability to run test suites in parallel.

lib/src/backend/live_test_controller.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.
 
 library test.backend.live_test_controller;
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'package:stack_trace/stack_trace.dart';
@@ skipping 141 matching lines @@
   }
 
   /// Emits a line printed by the test over [LiveTest.onPrint].
   void print(String line) => _onPrintController.add(line);
 
   /// A wrapper for [_onRun] that ensures that it follows the guarantees for
   /// [LiveTest.run].
   Future _run() {
     if (_runCalled) {
       throw new StateError("LiveTest.run() may not be called more than once.");
+    } else if (_isClosed) {
+      throw new StateError("LiveTest.run() may not be called for a closed "
+          "test.");
     }
     _runCalled = true;
 
     _onRun();
     return liveTest.onComplete;
   }
 
   /// A wrapper for [_onClose] that ensures that all controllers are closed.
   Future _close() {
     if (_isClosed) return completer.future;
 
     _onStateChangeController.close();
     _onErrorController.close();
 
     if (_runCalled) {
       _onClose();
     } else {
       completer.complete();
     }
 
     return completer.future;
   }
 }