add a const ctor to Converter

tests/lib/convert/codec1_test.dart

Index: tests/lib/convert/codec1_test.dart
diff --git a/tests/lib/convert/codec1_test.dart b/tests/lib/convert/codec1_test.dart
index 6132c9616b87ef7db401a6b8e5f96c09a425b4ce..5b5d9b3bf08d1621d8644a45307c878a46bf7a68 100644
--- a/tests/lib/convert/codec1_test.dart
+++ b/tests/lib/convert/codec1_test.dart
@@ -9,15 +9,19 @@ 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);
 }