Introduce class Sink<T>.

sdk/lib/convert/chunked_conversion.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;
 
 typedef void _ChunkedConversionCallback<T>(T accumulated);
 
 /**
  * A [ChunkedConversionSink] is used to transmit data more efficiently between
  * two converters during chunked conversions.
  *
+ * The basic `ChunkedConversionSink` is just a [Sink], and converters should
+ * work with a plain `Sink`, but may work more efficiently with certain
+ * specialized types of `ChunkedConversionSink`.
+ *
  * It is recommended that implementations of `ChunkedConversionSink` extends
  * this class, to inherit any further methods that may be added to the class.
  */
-abstract class ChunkedConversionSink<T> {
+abstract class ChunkedConversionSink<T> implements Sink<T> {
   ChunkedConversionSink();
   factory ChunkedConversionSink.withCallback(
       void callback(List<T> accumulated)) = _SimpleCallbackSink;
 
   /**
    * Adds chunked data to this sink.
    *
    * This method is also used when converters are used as [StreamTransformer]s.
    */
   void add(T chunk);
@@ skipping 46 matching lines @@
   final EventSink<T> _eventSink;
 
   /**
    * The input sink for new data. All data that is received with
    * [handleData] is added into this sink.
    */
   ChunkedConversionSink _chunkedSink;
 
   _ConverterStreamEventSink(Converter converter, EventSink<T> sink)
       : this._eventSink = sink,
-        _chunkedSink =
-            converter.startChunkedConversion(new _EventSinkAdapter(sink));
+        _chunkedSink = converter.startChunkedConversion(sink);
 
   void add(S o) => _chunkedSink.add(o);
   void addError(Object error, [StackTrace stackTrace]) {
     _eventSink.addError(error, stackTrace);
   }
   void close() => _chunkedSink.close();
 }