Refactor reifier to make it less dart_codegen specific.

test/dart_codegen/expect/async/schedule_microtask.dart

Index: test/dart_codegen/expect/async/schedule_microtask.dart
diff --git a/test/dart_codegen/expect/async/schedule_microtask.dart b/test/dart_codegen/expect/async/schedule_microtask.dart
index d15ff2a8bd20312401488c0030906a12e24dcdf7..5b50f93ead5737903a49d2580015c6b3b33604c1 100644
--- a/test/dart_codegen/expect/async/schedule_microtask.dart
+++ b/test/dart_codegen/expect/async/schedule_microtask.dart
@@ -30,19 +30,19 @@ _isInCallbackLoop = true;
 }
  void _scheduleAsyncCallback(callback) {
 if (_nextCallback == null) {
-  _nextCallback = _lastCallback = new _AsyncCallbackEntry(DEVC$RT.cast(callback, dynamic, __t20, "CompositeCast", """line 66, column 61 of dart:async/schedule_microtask.dart: """, callback is __t20, false));
+  _nextCallback = _lastCallback = new _AsyncCallbackEntry(DEVC$RT.cast(callback, dynamic, _AsyncCallback, "CompositeCast", """line 66, column 61 of dart:async/schedule_microtask.dart: """, callback is _AsyncCallback, false));
    if (!_isInCallbackLoop) {
     _AsyncRun._scheduleImmediate(_asyncRunCallback);
     }
   }
  else {
-  _AsyncCallbackEntry newEntry = new _AsyncCallbackEntry(DEVC$RT.cast(callback, dynamic, __t20, "CompositeCast", """line 71, column 60 of dart:async/schedule_microtask.dart: """, callback is __t20, false));
+  _AsyncCallbackEntry newEntry = new _AsyncCallbackEntry(DEVC$RT.cast(callback, dynamic, _AsyncCallback, "CompositeCast", """line 71, column 60 of dart:async/schedule_microtask.dart: """, callback is _AsyncCallback, false));
    _lastCallback.next = newEntry;
    _lastCallback = newEntry;
   }
 }
  void _schedulePriorityAsyncCallback(callback) {
-_AsyncCallbackEntry entry = new _AsyncCallbackEntry(DEVC$RT.cast(callback, dynamic, __t20, "CompositeCast", """line 84, column 55 of dart:async/schedule_microtask.dart: """, callback is __t20, false));
+_AsyncCallbackEntry entry = new _AsyncCallbackEntry(DEVC$RT.cast(callback, dynamic, _AsyncCallback, "CompositeCast", """line 84, column 55 of dart:async/schedule_microtask.dart: """, callback is _AsyncCallback, false));
  if (_nextCallback == null) {
   _scheduleAsyncCallback(callback);
    _lastPriorityCallback = _lastCallback;
@@ -68,4 +68,3 @@ if (identical(_ROOT_ZONE, Zone.current)) {
 }
  class _AsyncRun {external static void _scheduleImmediate(void callback());
 }
- typedef void __t20();