Add a type kind to TypedSelector. A typed selector can either be

tests/language/mixin_this_use_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.
+
+// Test that [:this:] in a class A used as a mixin in class D knows it can be an
+// instance of D.
+
+class A {
+  foo() => bar(); // Implicit use of [:this:]
+  bar() => 42;
+}
+
+class B {}
+
+typedef C = B with A;
+
+class D extends C {
+  bar() => 54;
+}
+
+class E extends A {
+  bar() => 68;
+}
+
+main() {
+  Expect.equals(54, new D().foo());
+  Expect.equals(68, new E().foo());
+}