Avoid runtime type errors in checked mode for ChunkedConverters.

sdk/lib/convert/converter.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;
 
 /**
  * A [Converter] converts data from one representation into another.
  *
  * It is recommended that implementations of `Converter` extend this class,
@@ skipping 34 matching lines @@
         stream,
         (EventSink sink) => new _ConverterStreamEventSink(this, sink));
   }
 }
 
 /**
  * Fuses two converters.
  *
  * For a non-chunked conversion converts the input in sequence.
  */
-class _FusedConverter<S, M, T> extends Converter<S, T> {
+class _FusedConverter<S, M, T> extends Converter<S, T>
+    implements ChunkedConverter<S, T, S, T> {
   final Converter<S, M> _first;
   final Converter<M, T> _second;
 
   _FusedConverter(this._first, this._second);
 
   T convert(S input) => _second.convert(_first.convert(input));
 
   Sink/*<S>*/ startChunkedConversion(Sink/*<T>*/ sink) {
     return _first.startChunkedConversion(_second.startChunkedConversion(sink));
   }
 }