Added a way to let the outer event loop run periodically so that things like

pkg/unittest/test/breath_test.dart

Index: pkg/unittest/test/breath_test.dart
===================================================================
--- pkg/unittest/test/breath_test.dart	(revision 0)
+++ pkg/unittest/test/breath_test.dart	(revision 0)
@@ -0,0 +1,34 @@
+// Copyright (c) 2012, 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 'package:unittest/unittest.dart';
+
+main() {
+  // Test the sync test 'breath' feature of unittest.
+
+  group('breath', () {
+    var sentinel = 0;
+    var start;
+
+    test('initial', () {
+      Timer.run(() { sentinel = 1; });
+    });
+
+    test('starve', () {
+      start = new DateTime.now().millisecondsSinceEpoch;
+      var now;
+      do {
+        expect(sentinel, 0);
+        now = new DateTime.now().millisecondsSinceEpoch;
+      } while (now - start <= BREATH_INTERVAL);
+    });
+
+    test('breathed', () {
+      expect(sentinel, 1);
+    });
+  });
+}
+