Rationalize coercions

test/dart_codegen/expect/convert/converter.dart

 part of dart.convert;
  abstract class Converter<S, T> implements StreamTransformer {const Converter();
  T convert(S input);
  Converter<S, dynamic> fuse(Converter<T, dynamic> other) {
   return new _FusedConverter<S, T, dynamic>(this, other);
   }
  ChunkedConversionSink startChunkedConversion(Sink sink) {
   throw new UnsupportedError("This converter does not support chunked conversions: $this");
   }
  Stream bind(Stream source) {
   return new Stream.eventTransformed(source, (EventSink sink) => new _ConverterStreamEventSink(this, sink));
   }
 }
  class _FusedConverter<S, M, T> extends Converter<S, T> {final Converter _first;
  final Converter _second;
  _FusedConverter(this._first, this._second);
- T convert(S input) => ((__x4) => DEVC$RT.cast(__x4, dynamic, T, "CastGeneral", """line 58, column 25 of dart:convert/converter.dart: """, __x4 is T, false))(_second.convert(_first.convert(input)));
+ T convert(S input) => ((__x4) => DEVC$RT.cast(__x4, dynamic, T, "CompositeCast", """line 58, column 25 of dart:convert/converter.dart: """, __x4 is T, false))(_second.convert(_first.convert(input)));
  ChunkedConversionSink startChunkedConversion(Sink sink) {
 return _first.startChunkedConversion(_second.startChunkedConversion(sink));
 }
 }