Fix bug in BytesBuilder. Add tests.

sdk/lib/io/bytes_builder.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;
 
 /**
  * Builds a list of bytes, allowing bytes and lists of bytes to be added at the
  * end.
  *
@@ skipping 142 matching lines @@
 
 class _BytesBuilder implements BytesBuilder {
   int _length = 0;
   final List<Uint8List> _chunks = [];
 
   void add(List<int> bytes) {
     Uint8List typedBytes;
     if (bytes is Uint8List) {
       typedBytes = bytes;
     } else {
-      bytes = new Uint8List.fromList(bytes);
+      typedBytes = new Uint8List.fromList(bytes);
     }
     _chunks.add(typedBytes);
     _length += typedBytes.length;
   }
 
   void addByte(int byte) { add([byte]); }
 
   List<int> takeBytes() {
     if (_chunks.length == 0) return new Uint8List(0);
     if (_chunks.length == 1) {
@@ skipping 26 matching lines @@
 
   bool get isEmpty => _length == 0;
 
   bool get isNotEmpty => _length != 0;
 
   void clear() {
     _length = 0;
     _chunks.clear();
   }
 }