Add Duration.inMicroseconds and add Stopwatch.elapsed.

tests/corelib/stopwatch2_test.dart

+// Copyright (c) 2013, 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.
+
+// Dart test program for elapsed getters in stopwatch support.
+
+main() {
+  Stopwatch sw = new Stopwatch()..start();
+  while (sw.elapsedMilliseconds < 2) {
+    /* do nothing. */
+  }
+  sw.stop();
+  Expect.equals(sw.elapsedMicroseconds, sw.elapsed.inMicroseconds);
+  Expect.equals(sw.elapsedMilliseconds, sw.elapsed.inMilliseconds);
+}