Refactor reifier to make it less dart_codegen specific.

test/dart_codegen/expect/convert/json.dart

 part of dart.convert;
  class JsonUnsupportedObjectError extends Error {final unsupportedObject;
  final cause;
  JsonUnsupportedObjectError(this.unsupportedObject, {
   this.cause}
 );
  String toString() {
   if (cause != null) {
     return "Converting object to an encodable object failed.";
     }
@@ skipping 23 matching lines @@
 }
  String encode(Object value, {
 toEncodable(var object)}
 ) {
 if (toEncodable == null) toEncodable = _toEncodable;
  if (toEncodable == null) return encoder.convert(value);
  return new JsonEncoder(DEVC$RT.cast(toEncodable, __t8, __t6, "CompositeCast", """line 142, column 28 of dart:convert/json.dart: """, toEncodable is __t6, false)).convert(value);
 }
  JsonEncoder get encoder {
 if (_toEncodable == null) return const JsonEncoder();
- return new JsonEncoder(DEVC$RT.cast(_toEncodable, __t8, __t6, "CompositeCast", """line 147, column 28 of dart:convert/json.dart: """, _toEncodable is __t6, false));
+ return new JsonEncoder(DEVC$RT.cast(_toEncodable, _ToEncodable, __t6, "CompositeCast", """line 147, column 28 of dart:convert/json.dart: """, _toEncodable is __t6, false));
 }
  JsonDecoder get decoder {
 if (_reviver == null) return const JsonDecoder();
  return new JsonDecoder(_reviver);
 }
 }
  class JsonEncoder extends Converter<Object, String> {final String indent;
  final Function _toEncodable;
  const JsonEncoder([Object toEncodable(Object nonSerializable)]) : this.indent = null, this._toEncodable = toEncodable;
  const JsonEncoder.withIndent(this.indent, [Object toEncodable(Object nonSerializable)]) : this._toEncodable = toEncodable;
@@ skipping 484 matching lines @@
 for (int i = 0; i < indentLength; i++) {
 writeByte(indent[i]);
 }
 }
 }
 }
 }
  typedef Object __t6(Object __u7);
  typedef dynamic __t8(dynamic __u9);
  typedef dynamic __t11(Object __u12);