Add a VmListener and IsolateTest class for running tests in other isolates.

lib/src/live_test.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 unittest.live_test;
 
 import 'dart:async';
 
 import 'state.dart';
 import 'suite.dart';
@@ skipping 83 matching lines @@
 
   /// Signals that this test should start running as soon as possible.
   ///
   /// A test may not start running immediately for various reasons specific to
   /// the means by which it's defined. Until it starts running, [state] will
   /// continue to be marked [Status.pending].
   ///
   /// This returns the same [Future] as [onComplete]. It may not be called more
   /// than once.
   Future run();
+
+  /// Signals that this test should stop emitting events and release any
+  /// resources it may have allocated.
+  ///
+  /// Once [close] is called, [onComplete] will complete if it hasn't already
+  /// and [onStateChange] and [onError] will close immediately. This means that,
+  /// if the test was running at the time [close] is called, it will never emit
+  /// a [Status.complete] state-change event.
+  ///
+  /// This doesn't automatically happen after the test completes because there
+  /// may be more asynchronous work going on in the background that could
+  /// produce new errors.
+  Future close();
 }