Rationalize coercions

test/dart_codegen/expect/convert/codec.dart

 part of dart.convert;
  abstract class Codec<S, T> {const Codec();
  T encode(S input) => encoder.convert(input);
  S decode(T encoded) => decoder.convert(encoded);
  Converter<S, T> get encoder;
  Converter<T, S> get decoder;
  Codec<S, dynamic> fuse(Codec<T, dynamic> other) {
   return new _FusedCodec<S, T, dynamic>(this, other);
   }
  Codec<T, S> get inverted => new _InvertedCodec<T, S>(this);
 }
  class _FusedCodec<S, M, T> extends Codec<S, T> {final Codec<S, M> _first;
  final Codec<M, T> _second;
  Converter<S, T> get encoder => ((__x2) => DEVC$RT.cast(__x2, DEVC$RT.type((Converter<S, dynamic> _) {
 }
 ), DEVC$RT.type((Converter<S, T> _) {
 }
-), "CastDynamic", """line 87, column 34 of dart:convert/codec.dart: """, __x2 is Converter<S, T>, false))(_first.encoder.fuse(_second.encoder));
+), "CompositeCast", """line 87, column 34 of dart:convert/codec.dart: """, __x2 is Converter<S, T>, false))(_first.encoder.fuse(_second.encoder));
  Converter<T, S> get decoder => ((__x3) => DEVC$RT.cast(__x3, DEVC$RT.type((Converter<T, dynamic> _) {
 }
 ), DEVC$RT.type((Converter<T, S> _) {
 }
-), "CastDynamic", """line 88, column 34 of dart:convert/codec.dart: """, __x3 is Converter<T, S>, false))(_second.decoder.fuse(_first.decoder));
+), "CompositeCast", """line 88, column 34 of dart:convert/codec.dart: """, __x3 is Converter<T, S>, false))(_second.decoder.fuse(_first.decoder));
  _FusedCodec(this._first, this._second);
 }
  class _InvertedCodec<T, S> extends Codec<T, S> {final Codec<S, T> _codec;
  _InvertedCodec(Codec<S, T> codec) : _codec = codec;
  Converter<T, S> get encoder => _codec.decoder;
  Converter<S, T> get decoder => _codec.encoder;
  Codec<S, T> get inverted => _codec;
 }