Add more mixin tests and start rejecting illegal syntax.

tests/language/mixin_mixin_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.
+
+class S { }
+class M1 { foo() => 42; }
+typedef M2 = S with M1;
+typedef C = S with M2;
+
+main() {
+  var c = new C();
+  Expect.isTrue(c is S);
+  Expect.isTrue(c is M1);
+  Expect.isTrue(c is M2);
+  Expect.isTrue(c is C);
+  Expect.equals(42, c.foo());
+}