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

tests/language_strong/type_no_hoisting_test.dart

-// compile options: --no-hoist-signature-types --no-hoist-instance-creation --name-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) {
-    return x;
-  }
-
-  T x;
-}
-
-class B extends A<int> {
-  B(int x, int z) : super(x, z);
-  B.make() : super.make();
-  void f(int x) {}
-  static int g(int x) {
-    return x;
-  }
-}
-
-class C {
-  C(this.x, int z);
-  void f(int x) {}
-  static int g(int x) {
-    return x;
-  }
-
-  int x = 0;
-}
-
-typedef void ToVoid<T>(T x);
-typedef T Id<T>(T x);
-
-void main() {
-  {
-    A<String> a = new A<String>("hello", "world");
-    Expect.isTrue(new A<String>.make() is! A<int>);
-    Expect.isTrue(new A.make() is A);
-    Expect.isTrue(a is! A<int>);
-    Expect.isTrue(a is A<String>);
-    Expect.isTrue(a.f is ToVoid<String>);
-    Expect.isTrue(A.g is Id<String>);
-  }
-  {
-    B b = new B(0, 1);
-    Expect.isTrue(new B.make() is B);
-    Expect.isTrue(new B.make() is A<int>);
-    Expect.isTrue(b is B);
-    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>);
-  }
-}