Add new features to package:async.

test/utils.dart

Index: test/utils.dart
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+// 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.
+
+/// Helper utilities for testing.
+library async.test.util;
+
+import "dart:async";
+
+/// A zero-millisecond timer should wait until after all microtasks.
+Future flushMicrotasks() => new Future.delayed(Duration.ZERO);
+
+/// A generic unreachable callback function.
+///
+/// Returns a function that fails the test if it is ever called.
+unreachable(String name) => ([a, b]) => fail("Unreachable: $name");
+
+/// A badly behaved stream which throws if it's ever listened to.
+///
+/// Can be used to test cases where a stream should not be used.
+class UnusableStream extends Stream {
+  listen(onData, {onError, onDone, cancelOnError}) {
+    throw new UnimplementedError("Gotcha!");
+  }
+}