Rework type assignability, subtype, and "more specific than" checking.

tests/language/regress_22976_test.dart

 // Copyright (c) 2015, 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.
 
-// Regression test for http://dartbug.com/22487
+// Regression test for issue 22976.
 
-import 'package:expect/expect.dart';
-
-divIsInt(a, b) => (a / b) is int;
+class A<T> {}
+class B<T> implements A<T> {}
+class C<S, T> implements B<S>, A<T> {}
 
 main() {
-  Expect.isFalse((divIsInt)(10, 3));
+  A<int> a0 = new C<int, String>(); /// 01: ok
+  A<int> a1 = new C<String, int>(); /// 02: ok
 }