Change "typedef" to "class" in core library and related tests.

tests/language/mixin_illegal_object_test.dart

Index: tests/language/mixin_illegal_object_test.dart
diff --git a/tests/language/mixin_illegal_object_test.dart b/tests/language/mixin_illegal_object_test.dart
index ef11ef1e1aa57f8d9209ad99d2559e76924fc761..f3dae74ee082734ab0e0406707238d97e30f2742 100644
--- a/tests/language/mixin_illegal_object_test.dart
+++ b/tests/language/mixin_illegal_object_test.dart
@@ -6,7 +6,7 @@ class C0 extends Object
 with Object                       /// 01: compile-time error
 { }
 
-typedef C1 = Object with Object;  /// 02: compile-time error
+class C1 = Object with Object;  /// 02: compile-time error
 
 main() {
   new C0();