Add Stream.periodic.

tests/lib/async/stream_periodic3_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.
+
+// Test merging streams.
+library dart.test.stream_from_iterable;
+
+import "dart:async";
+import '../../../pkg/unittest/lib/unittest.dart';
+
+main() {
+  test("stream-periodic3", () {
+    Stopwatch watch = new Stopwatch()..start();
+    Stream stream = new Stream.periodic(const Duration(milliseconds: 1),
+                                        (x) => x);
+    stream.take(10).listen((_) { }, onDone: expectAsync0(() {
+      int microsecs = watch.elapsedMicroseconds;
+      // Give it some slack. The Stopwatch is more precise than the timers.
+      int millis = (microsecs + 200) ~/ 1000;
+      expect(millis, greaterThan(10));
+    }));
+  });
+}