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

tests/language/mixin_is_test.dart

Index: tests/language/mixin_is_test.dart
diff --git a/tests/language/mixin_is_test.dart b/tests/language/mixin_is_test.dart
index 7e8d08014302a136c5d4561b19c735b739d90737..2e1cadaf2ebe10b53b95bf52782df4840d8577ad 100644
--- a/tests/language/mixin_is_test.dart
+++ b/tests/language/mixin_is_test.dart
@@ -8,14 +8,14 @@ class S { }
 class M1 { }
 class M2 { }
 
-typedef C = S with M1;
-typedef D = S with M1, M2;
-typedef E = S with M2, M1;
+class C = S with M1;
+class D = S with M1, M2;
+class E = S with M2, M1;
 class F extends E { }
 
-typedef C_ = S with M1;
-typedef D_ = S with M1, M2;
-typedef E_ = S with M2, M1;
+class C_ = S with M1;
+class D_ = S with M1, M2;
+class E_ = S with M2, M1;
 class F_ extends E_ { }
 
 main() {