Fix one final strong-mode warning.

lib/src/utils.dart

 // Copyright (c) 2012, 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';
 import 'dart:convert';
 import 'dart:typed_data';
 
 import 'byte_stream.dart';
 
@@ skipping 99 matching lines @@
 /// [stream] is done. Unlike [store], [sink] remains open after [stream] is
 /// done.
 Future writeStreamToSink(Stream stream, EventSink sink) {
   var completer = new Completer();
   stream.listen(sink.add,
       onError: sink.addError,
       onDone: () => completer.complete());
   return completer.future;
 }
 
-/// Returns a [Future] that asynchronously completes to `null`.
-Future get async => new Future.value();
-
 /// A pair of values.
 class Pair<E, F> {
   E first;
   F last;
 
   Pair(this.first, this.last);
 
   String toString() => '($first, $last)';
 
   bool operator==(other) {
     if (other is! Pair) return false;
     return other.first == first && other.last == last;
   }
 
   int get hashCode => first.hashCode ^ last.hashCode;
 }
 
 /// Configures [future] so that its result (success or exception) is passed on
 /// to [completer].
 void chainToCompleter(Future future, Completer completer) {
   future.then(completer.complete, onError: completer.completeError);
 }