Make Asset.readAsString() async.

pkg/barback/lib/src/asset.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.asset;
 
 import 'dart:async';
 import 'dart:io';
 
 /// A blob of content.
@@ skipping 18 matching lines @@
   factory Asset.deserialize(data) {
     // TODO(rnystrom): Handle errors.
     switch (data[0]) {
       case "file": return new _FileAsset(new File(data[1])); break;
       case "string": return new _StringAsset(data[1]); break;
     }
   }
 
   /// Returns the contents of the asset as a string.
   // TODO(rnystrom): Figure out how binary assets should be handled.
-  String readAsString();
+  Future<String> readAsString();
 
   /// Streams the contents of the asset.
   Stream<List<int>> read();
 
   /// Serializes this [Asset] to an object that can be sent across isolates
   /// and passed to [deserialize].
   Object serialize();
 }
 
 /// An asset backed by a file on the local file system.
 class _FileAsset implements Asset {
   final File _file;
   _FileAsset(this._file);
 
-  String readAsString() => _file.readAsStringSync();
+  Future<String> readAsString() => _file.readAsString();
   Stream<List<int>> read() => _file.openRead();
 
   String toString() => 'File "${_file.path}"';
 
   Object serialize() => ["file", _file.path];
 }
 
 /// An asset whose data is stored in a string.
 // TODO(rnystrom): Have something similar for in-memory binary assets.
 class _StringAsset implements Asset {
   final String _contents;
 
   _StringAsset(this._contents);
 
-  String readAsString() => _contents;
+  Future<String> readAsString() => new Future.value(_contents);
 
   // TODO(rnystrom): Implement this and handle encoding.
   Stream<List<int>> read() => throw new UnimplementedError();
 
   String toString() {
     // Don't show the whole string if it's long.
     var contents = _contents;
     if (contents.length > 40) {
       contents = contents.substring(0, 20) + " ... " +
                  contents.substring(contents.length - 20);
     }
 
     contents = _escape(contents);
     return 'String "$contents"';
   }
 
   Object serialize() => ["string", _contents];
 
   String _escape(String string) {
     return string
         .replaceAll("\"", r'\"')
         .replaceAll("\n", r"\n")
         .replaceAll("\r", r"\r")
         .replaceAll("\t", r"\t");
   }
 }