Add Stream.empty.

tests/lib/async/stream_empty_test.dart

+// Copyright (c) 2015, 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 empty stream.
+import "package:expect/expect.dart";
+import "dart:async";
+import 'package:async_helper/async_helper.dart';
+
+main() async {
+  unreachable([a,b]) { throw "UNREACHABLE"; }
+  int tick = 0;
+  ticker() { tick++; }
+
+  asyncStart();
+
+  Stream<int> s = const Stream<int>.empty();  // Is const constructor.
+  Expect.isFalse(s is Stream<String>);  // Respects type parameter.
+  StreamSubscription<int> sub =
+     s.listen(unreachable, onError: unreachable, onDone: ticker);
+  Expect.isFalse(sub is StreamSubscription<String>);  // Type parameter in sub.
+
+  // Doesn't do callback in response to listen.
+  Expect.equals(tick, 0);
+  await flushMicrotasks();
+  // Completes eventually.
+  Expect.equals(tick, 1);
+
+  // It's a broadcast stream - can listen twice.
+  Expect.isTrue(s.isBroadcast);
+  StreamSubscription<int> sub2 =
+     s.listen(unreachable, onError: unreachable, onDone: unreachable);
+  // respects pause.
+  sub2.pause();
+  await flushMicrotasks();
+  // respects cancel.
+  sub2.cancel();
+  await flushMicrotasks();
+  Expect.equals(tick, 1);
+  // Still not complete.
+
+  StreamSubscription<int> sub3 =
+     s.listen(unreachable, onError: unreachable, onDone: ticker);
+  // respects pause.
+  sub3.pause();
+  Expect.equals(tick, 1);
+  await flushMicrotasks();
+  // Doesn't complete while paused.
+  Expect.equals(tick, 1);
+  sub3.resume();
+  await flushMicrotasks();
+  // Now completed.
+  Expect.equals(tick, 2);
+
+  asyncEnd();
+}
+
+Future flushMicrotasks() => new Future.delayed(Duration.ZERO);