Refactor reifier to make it less dart_codegen specific.

test/dart_codegen/expect/convert/json.dart

Index: test/dart_codegen/expect/convert/json.dart
diff --git a/test/dart_codegen/expect/convert/json.dart b/test/dart_codegen/expect/convert/json.dart
index 9196fc4d9857f56e0585d29fdedbcd2ba14c8ee2..5323444b562d57ad2840b8ce289800ce46043238 100644
--- a/test/dart_codegen/expect/convert/json.dart
+++ b/test/dart_codegen/expect/convert/json.dart
@@ -41,7 +41,7 @@ if (toEncodable == null) toEncodable = _toEncodable;
 }
  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();