fix Utf8Decoder hook.

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 317 matching lines @@
    * If [end] is omitted, it defaults to `codeUnits.length`.
    *
    * If the [codeUnits] start with a leading [UNICODE_BOM_CHARACTER_RUNE] this
    * character is discarded.
    */
   String convert(List<int> codeUnits, [int start = 0, int end]) {
     // Allow the implementation to intercept and specialize based on the type
     // of codeUnits.
     String result = _convertIntercepted(_allowMalformed, codeUnits, start, end);
     if (result != null) {
-      return null;
+      return result;
     }
 
     int length = codeUnits.length;
     RangeError.checkValidRange(start, end, length);
     if (end == null) end = length;
     StringBuffer buffer = new StringBuffer();
     _Utf8Decoder decoder = new _Utf8Decoder(buffer, _allowMalformed);
     decoder.convert(codeUnits, start, end);
     decoder.close();
     return buffer.toString();
@@ skipping 213 matching lines @@
       }
       break loop;
     }
     if (expectedUnits > 0) {
       _value = value;
       _expectedUnits = expectedUnits;
       _extraUnits = extraUnits;
     }
   }
 }