Remove AsyncError with Expando.

sdk/lib/io/string_transformer.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.
 
 part of dart.io;
 
 /**
  * String encodings.
  */
 class Encoding {
@@ skipping 297 matching lines @@
 
   int _decodeByte(int byte) => ((byte & 0xFF) == byte) ? byte : -1;
 }
 
 
 abstract class _SingleByteEncoder
     extends StreamEventTransformer<String, List<int>> {
   void handleData(String data, EventSink<List<int>> sink) {
     var bytes = _encode(data);
     if (bytes == null) {
-      sink.addError(
-          new AsyncError(
-              new FormatException("Invalid character for encoding")));
+      sink.addError(new FormatException("Invalid character for encoding"));
       sink.close();
     } else {
       sink.add(bytes);
     }
   }
 
   List<int> _encode(String string);
 }
 
 
@@ skipping 34 matching lines @@
 
 // Utility class for decoding Windows current code page data delivered
 // as a stream of bytes.
 class _WindowsCodePageDecoder extends StreamEventTransformer<List<int>, String> {
   void handleData(List<int> data, EventSink<String> sink) {
     sink.add(_decodeBytes(data));
   }
 
   external static String _decodeBytes(List<int> bytes);
 }