add a const ctor to Converter

tests/lib/convert/codec1_test.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.
 
 import 'dart:convert';
 
 import 'package:expect/expect.dart';
 
 class MyCodec extends Codec<int, String> {
   const MyCodec();
 
-  Converter<int, String> get encoder => new IntStringConverter();
-  Converter<String, int> get decoder => new StringIntConverter();
+  final Converter<int, String> encoder = const IntStringConverter();
+  final Converter<String, int> decoder = const StringIntConverter();
 }
 
 class IntStringConverter extends Converter<int, String> {
+  const IntStringConverter();
+
   String convert(int i) => i.toString();
 }
 
 class StringIntConverter extends Converter<String, int> {
+  const StringIntConverter();
+
   int convert(String str) => int.parse(str);
 }
 
 class MyCodec2 extends Codec<int, String> {
   const MyCodec2();
 
   Converter<int, String> get encoder => new IntStringConverter2();
   Converter<String, int> get decoder => new StringIntConverter2();
 }
 
@@ skipping 29 matching lines @@
 
   inverted = TEST_CODEC2.inverted;
   fused = TEST_CODEC2.fuse(inverted);
   Expect.equals(499, fused.encode(0));
   Expect.equals(499, fused.decode(0));
 
   fused = TEST_CODEC.fuse(inverted);
   Expect.equals(405, fused.encode(5));
   Expect.equals(101, fused.decode(2));
 }