fix barback and polymer build warnings

pkg/barback/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 barback.utils;
 
 import 'dart:async';
 
 /// A pair of values.
 class Pair<E, F> {
@@ skipping 157 matching lines @@
 /// 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));
 }
 
-/// Like [new Future], but avoids issue 11911 by using [new Future.value] under
+/// Like `new Future`, but avoids issue 11911 by using `new Future.value` under
 /// the covers.
+// TODO(jmesserly): doc comment changed to due 14601.

[Jennifer Messerly, 2013/10/30 00:59:29]

https://code.google.com/p/dart/issues/detail?id=14601

 Future newFuture(callback()) => new Future.value().then((_) => callback());
 
 /// Returns a buffered stream that will emit the same values as the stream
 /// returned by [future] once [future] completes. If [future] completes to an
 /// error, the return value will emit that error and then close.
 Stream futureStream(Future<Stream> future) {
   var controller = new StreamController(sync: true);
   future.then((stream) {
     stream.listen(
         controller.add,
         onError: controller.addError,
         onDone: controller.close);
   }).catchError((e, stackTrace) {
     controller.addError(e, stackTrace);
     controller.close();
   });
   return controller.stream;
 }