Add Stream.periodic.

tests/lib/async/stream_periodic_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-periodic1", () {
+    Stream stream = new Stream.periodic(const Duration(milliseconds: 1));
+    int receivedCount = 0;
+    var subscription;
+    subscription = stream.listen(expectAsync1((data) {
+      expect(data, isNull);
+      receivedCount++;
+      if (receivedCount == 5) subscription.cancel();
+    }, count: 5));
+  });
+}