add a const ctor to Converter

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.
  *
  * *Converters are still experimental and are subject to change without notice.*
  *
  */
 abstract class Converter<S, T> implements StreamTransformer {
+  const Converter();
+
   /**
    * Converts [input] and returns the result of the conversion.
    */
   T convert(S input);
 
   /**
    * Fuses `this` with [other].
    *
    * Encoding with the resulting converter is equivalent to converting with
    * `this` before converting with `other`.
@@ skipping 26 matching lines @@
   final Converter _second;
 
   _FusedConverter(this._first, this._second);
 
   T convert(S input) => _second.convert(_first.convert(input));
 
   ChunkedConversionSink startChunkedConversion(ChunkedConversionSink sink) {
     return _first.startChunkedConversion(_second.startChunkedConversion(sink));
   }
 }