Add StreamChannel and MultiChannel.

test/utils.dart

Index: test/utils.dart
diff --git a/test/utils.dart b/test/utils.dart
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+++ b/test/utils.dart
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+// Copyright (c) 2016, 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.
+
+import 'dart:async';
+
+/// Returns a [Future] that completes after pumping the event queue [times]
+/// times.
+///
+/// By default, this should pump the event queue enough times to allow any code
+/// to run, as long as it's not waiting on some external event.
+Future pumpEventQueue([int times=20]) {
+  if (times == 0) return new Future.value();
+  // Use [new Future] future to allow microtask events to finish. The [new
+  // Future.value] constructor uses scheduleMicrotask itself and would therefore
+  // not wait for microtask callbacks that are scheduled after invoking this
+  // method.
+  return new Future(() => pumpEventQueue(times - 1));
+}
+