Use stream matchers to unflake the mac OS watcher tests.

pkg/watcher/lib/src/utils.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.
 
 library watcher.utils;
 
 import 'dart:async';
 import 'dart:io';
 import 'dart:collection';
 
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     controller = new StreamController(
         sync: true, onListen: onListen, onCancel: onCancel);
   }
   return controller.stream;
 }
 
 /// Like [new Future], but avoids around issue 11911 by using [new Future.value]
 /// under the covers.
 Future newFuture(callback()) => new Future.value().then((_) => callback());
 
+/// 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();
+  // We use a delayed future to allow microtask events to finish. The
+  // Future.value or Future() constructors use scheduleMicrotask themselves and
+  // would therefore not wait for microtask callbacks that are scheduled after
+  // invoking this method.
+  return new Future.delayed(Duration.ZERO, () => pumpEventQueue(times - 1));
+}
+
 /// A stream transformer that batches all events that are sent at the same time.
 ///
 /// When multiple events are synchronously added to a stream controller, the
 /// [StreamController] implementation uses [scheduleMicrotask] to schedule the
 /// asynchronous firing of each event. In order to recreate the synchronous
 /// batches, this collates all the events that are received in "nearby"
 /// microtasks.
 class BatchedStreamTransformer<T> implements StreamTransformer<T, List<T>> {
   Stream<List<T>> bind(Stream<T> input) {
     var batch = new Queue();
@@ skipping 10 matching lines @@
       });
     }, handleDone: (sink) {
       if (batch.isNotEmpty) {
         sink.add(batch.toList());
         batch.clear();
       }
       sink.close();
     }).bind(input);
   }
 }