Make _Utf8Encoder create its buffer through an external function.

runtime/lib/convert_patch.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.
 
+import "dart:typed_data";
+
 // JSON conversion.
 
 patch _parseJson(String json, reviver(var key, var value)) {
   _BuildJsonListener listener;
   if (reviver == null) {
     listener = new _BuildJsonListener();
   } else {
     listener = new _ReviverJsonListener(reviver);
   }
   new _JsonParser(json, listener).parse();
@@ skipping 532 matching lines @@
     String slice;
     int sliceEnd = position + 20;
     if (sliceEnd > source.length) {
       slice = "'${source.substring(position)}'";
     } else {
       slice = "'${source.substring(position, sliceEnd)}...'";
     }
     throw new FormatException("Unexpected character at $position: $slice");
   }
 }
+
+// UTF-8 conversion.
+
+patch class _Utf8Encoder {
+  /* patch */ static List<int> _createBuffer(int size) => new Uint8List(size);
+}