fix #27259, implement covariance checking for strong mode and DDC

tests/language_strong/type_hoisting_test.dart

-// compile options: --hoist-signature-types --hoist-instance-creation --hoist-type-tests
 // Copyright (c) 2016, 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.
 import "package:expect/expect.dart";
 
 class A<T> {
   A(this.x, T z);
   A.make();
   void f(T x) {}
   static String g(String x) {
@@ skipping 43 matching lines @@
     Expect.isTrue(b.f is ToVoid<int>);
     Expect.isTrue(B.g is Id<int>);
   }
   {
     C c = new C(0, 1);
     Expect.isTrue(c is C);
     Expect.isTrue(c.f is ToVoid<int>);
     Expect.isTrue(C.g is Id<int>);
   }
 }