Refactor reifier to make it less dart_codegen specific.

test/dart_codegen/expect/d/d.dart

Index: test/dart_codegen/expect/d/d.dart
diff --git a/test/dart_codegen/expect/d/d.dart b/test/dart_codegen/expect/d/d.dart
index 6aff212dcf06558652feb4193954e019138b12ba..66ef35375734b50569db77904c7be9cbd45d3184 100644
--- a/test/dart_codegen/expect/d/d.dart
+++ b/test/dart_codegen/expect/d/d.dart
@@ -8,10 +8,9 @@ void foo() {
   var x = f3(((__x0) => DEVC$RT.cast(__x0, dynamic, DDC$a$.A, "DynamicCast",
       """line 10, column 14 of test/dart_codegen/types/d.dart: """,
       __x0 is DDC$a$.A, true))(f4("""hello""")));
-  var y = f1(DEVC$RT.cast(f4, __t3, __t1, "CompositeCast",
-      """line 11, column 14 of test/dart_codegen/types/d.dart: """, f4 is __t1,
-      false));
+  var y = f1(DEVC$RT.cast(f4, __t1, DDC$a$.A2A, "CompositeCast",
+      """line 11, column 14 of test/dart_codegen/types/d.dart: """,
+      f4 is DDC$a$.A2A, false));
   var z = f2(f3);
 }
-typedef DDC$a$.A __t1(DDC$a$.A __u2);
-typedef dynamic __t3(dynamic __u4);
+typedef dynamic __t1(dynamic __u2);