Teach Selector.applies about mixins.

tests/language/mixin_override_regression_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.
 
-// Make sure the engine does not infer the wrong type for [:A.foo:].
-
-class A {
-  foo() => this;
+class C0 {
+  int m1() => 5;
+  int m2() => m1();
 }
 
-class B extends A {
+typedef C1 = Object with C0;
+
+class D {
+  int m1() => 7;
 }
 
+class E0 extends C0 with D {}
+class E1 extends C1 with D {}
+
 main() {
-  Expect.isTrue(new B().foo() is B);
+  Expect.equals(7, new E0().m2());
+  Expect.equals(7, new E1().m2());
 }