readd accidentally removed import.

pkg/polymer/lib/src/file_system/console.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.
 
 library console;
 
+import 'dart:async';
 import 'dart:convert';
 import 'dart:io';
 import 'package:polymer/src/file_system.dart';
 
 /** File system implementation for console VM (i.e. no browser). */
 class ConsoleFileSystem implements FileSystem {
 
   /** Pending futures for file write requests. */
   final _pending = <String, Future>{};
 
@@ skipping 20 matching lines @@
           .then((_) => file.close())
           .then((_) => buffer);
     }));
   }
 
   // TODO(jmesserly): do we support any encoding other than UTF-8 for Dart?
   Future<String> readText(String path) {
     return readTextOrBytes(path).then(UTF8.decode);
   }
 }