Add Stream.periodic.

tests/lib/async/stream_periodic5_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';
+
+watchMs(Stopwatch watch) {
+  int microsecs = watch.elapsedMicroseconds;
+  // Give it some slack. The Stopwatch is more precise than the timers. This
+  // means that we sometimes get 3995 microseconds instead of 4+ milliseconds.
+  // 200 microseconds should largely account for this discrepancy.
+  return (microsecs + 200) ~/ 1000;
+}
+
+main() {
+  test("stream-periodic4", () {
+    Stream stream = new Stream.periodic(const Duration(milliseconds: 5),
+                                        (x) => x);
+    Stopwatch watch = new Stopwatch()..start();
+    var subscription;
+    subscription = stream.take(10).listen((i) {
+      int ms = watchMs(watch);
+      watch.reset();
+      if (i == 2) {
+        Stopwatch watch2 = new Stopwatch()..start();
+        // Busy wait.
+        while (watch2.elapsedMilliseconds < 15) { }
+        // Make sure the stream can be paused when it has overdue events.
+        // We just busy waited for 15ms, even though the stream is supposed to
+        // emit events every 5ms.
+        subscription.pause();
+        watch.stop();
+        new Timer(const Duration(milliseconds: 150), () {
+          watch.start();
+          subscription.resume();
+        });
+      } else {
+        expect(ms, lessThan(100));
+      }
+    }, onDone: expectAsync0(() { }));
+  });
+}