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

tests/language/regress_21912_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 issue 21912.
+
+class A {}
+class B extends A {}
+
+typedef T Function2<S, T>(S z);
+typedef B AToB(A x);
+typedef A BToA(B x);
+
+void main() {
+  {
+    Function2<Function2<A, B>, Function2<B, A>> t1;
+    Function2<AToB, BToA> t2;
+    Function2<Function2<int, double>, Function2<int, double>> left;
+    left = t1; /// 01: ok
+    left = t2; /// 02: ok
+  }
+}