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

tests/language/regress_22976_test.dart

Index: tests/language/regress_22976_test.dart
diff --git a/tests/compiler/dart2js_extra/22487_test.dart b/tests/language/regress_22976_test.dart
similarity index 51%
copy from tests/compiler/dart2js_extra/22487_test.dart
copy to tests/language/regress_22976_test.dart
index 4bf5426b20e4dc4508044fb5aa0884afe5082e4a..67d69570b3f775d24e480c763b501f95d4f29786 100644
--- a/tests/compiler/dart2js_extra/22487_test.dart
+++ b/tests/language/regress_22976_test.dart
@@ -2,12 +2,13 @@
 // 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
 }