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

tests/language/mixin_implements_test.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import "package:expect/expect.dart";
 
 abstract class I0 {
   foo();
 }
 
 abstract class I1 {
   bar();
 }
 
 abstract class I2 implements I0, I1 {
 }
 
 class M {
   foo() => 42;
   bar() => 87;
 }
 
-typedef C0 = Object with M;
-typedef C1 = Object with M implements I0;
-typedef C2 = Object with M implements I1;
-typedef C3 = Object with M implements I0, I1;
-typedef C4 = Object with M implements I1, I0;
-typedef C5 = Object with M implements I2;
+class C0 = Object with M;
+class C1 = Object with M implements I0;
+class C2 = Object with M implements I1;
+class C3 = Object with M implements I0, I1;
+class C4 = Object with M implements I1, I0;
+class C5 = Object with M implements I2;
 
 main() {
   var c0 = new C0();
   Expect.equals(42, c0.foo());
   Expect.equals(87, c0.bar());
   Expect.isTrue(c0 is M);
   Expect.isFalse(c0 is I0);
   Expect.isFalse(c0 is I1);
   Expect.isFalse(c0 is I2);
 
@@ skipping 30 matching lines @@
   Expect.isFalse(c1 is I2);
 
   var c5 = new C5();
   Expect.equals(42, c5.foo());
   Expect.equals(87, c5.bar());
   Expect.isTrue(c5 is M);
   Expect.isTrue(c5 is I0);
   Expect.isTrue(c5 is I1);
   Expect.isTrue(c5 is I2);
 }