Refactor reifier to make it less dart_codegen specific.

test/dart_codegen/expect/async/stream_pipe.dart

Index: test/dart_codegen/expect/async/stream_pipe.dart
diff --git a/test/dart_codegen/expect/async/stream_pipe.dart b/test/dart_codegen/expect/async/stream_pipe.dart
index 104ba69d095c9d03055ae2c8a008c28819f6fb52..b468d50173c97212492d061b5c5c0808f17d26d1 100644
--- a/test/dart_codegen/expect/async/stream_pipe.dart
+++ b/test/dart_codegen/expect/async/stream_pipe.dart
@@ -130,7 +130,7 @@ sink._add(inputEvent);
  void _handleData(S inputEvent, _EventSink<T> sink) {
 T outputEvent;
  try {
-outputEvent = ((__x69) => DEVC$RT.cast(__x69, dynamic, T, "CompositeCast", """line 235, column 21 of dart:async/stream_pipe.dart: """, __x69 is T, false))(_transform(inputEvent));
+outputEvent = ((__x61) => DEVC$RT.cast(__x61, dynamic, T, "CompositeCast", """line 235, column 21 of dart:async/stream_pipe.dart: """, __x61 is T, false))(_transform(inputEvent));
 }
  catch (e, s) {
 _addErrorWithReplacement(sink, e, s);