Upgrade to the latest dart:convert.

tool/input_sdk/patch/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 argumentErrorValue, patch;
 import 'dart:_foreign_helper' show JS;
 import 'dart:_interceptors' show JSExtendableArray;
 import 'dart:_internal' show MappedIterable, ListIterable;
@@ skipping 379 matching lines @@
     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);
+  Converter<List<int>, dynamic/*=T*/> fuse/*<T>*/(
+      Converter<String, dynamic/*=T*/> next) {
+    return super.fuse/*<T>*/(next);
   }
 
   // Currently not intercepting UTF8 decoding.
   @patch
   static String _convertIntercepted(bool allowMalformed, List<int> codeUnits,
                                     int start, int end) {
     return null;  // This call was not intercepted.
   }
 }