Make JSON.fuse(UTF8) be more efficient by not creating intermediate string.

sdk/lib/_internal/compiler/js_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.
 
 // Patch file for dart:convert library.
 
 import 'dart:_js_helper' show patch;
 import 'dart:_foreign_helper' show JS;
 import 'dart:_interceptors' show JSExtendableArray;
 import 'dart:_internal' show MappedIterable;
@@ skipping 351 matching lines @@
   void close() {
     super.close();
     StringBuffer buffer = _stringSink;
     String accumulated = buffer.toString();
     buffer.clear();
     Object decoded = _parseJson(accumulated, _reviver);
     _sink.add(decoded);
     _sink.close();
   }
 }
+
+@patch class Utf8Decoder {
+  @patch
+  Converter<List<int>,dynamic> fuse(Converter<String, dynamic> next) {
+    return super.fuse(next);
+  }
+}