Migrate block 162 to Dart 2.0.

tests/language_2/type_propagation_assert_assignable_test.dart

 // Copyright (c) 2013, 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.
 // Check that type of the AssertAssignable is recomputed correctly.
 // VMOptions=--optimization-counter-threshold=10 --no-use-osr
 
 import "package:expect/expect.dart";
 
 class A {
   final p;
@@ skipping 27 matching lines @@
 }
 
 main() {
   final a = new A(new B(new A("haha", true), false), false);
 
   for (var i = 0; i < 20; i++) {
     Expect.isTrue(foo(a));
   }
   Expect.isTrue(foo(a));
 }