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

sdk/lib/convert/utf.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.convert;
 
 /** The Unicode Replacement character `U+FFFD` (�). */
 const int UNICODE_REPLACEMENT_CHARACTER_RUNE = 0xFFFD;
 
 /** The Unicode Byte Order Marker (BOM) character `U+FEFF`. */
@@ skipping 324 matching lines @@
     if (sink is StringConversionSink) {
       stringSink = sink;
     } else {
       stringSink = new StringConversionSink.from(sink);
     }
     return stringSink.asUtf8Sink(_allowMalformed);
   }
 
   // Override the base-classes bind, to provide a better type.
   Stream<String> bind(Stream<List<int>> stream) => super.bind(stream);
+
+  external Converter<List<int>,dynamic> fuse(Converter<String, dynamic> next);
 }
 
 // UTF-8 constants.
 const int _ONE_BYTE_LIMIT = 0x7f;   // 7 bits
 const int _TWO_BYTE_LIMIT = 0x7ff;  // 11 bits
 const int _THREE_BYTE_LIMIT = 0xffff;  // 16 bits
 const int _FOUR_BYTE_LIMIT = 0x10ffff;  // 21 bits, truncated to Unicode max.
 
 // UTF-16 constants.
 const int _SURROGATE_MASK = 0xF800;
@@ skipping 181 matching lines @@
       }
       break loop;
     }
     if (expectedUnits > 0) {
       _value = value;
       _expectedUnits = expectedUnits;
       _extraUnits = extraUnits;
     }
   }
 }