Migrate block 162 to Dart 2.0.

tests/language_strong/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;
-  final _b;
-
-  b() {
-    try {
-      return _b;
-    } catch (e) {}
-  }
-
-  A(this.p, this._b);
-}
-
-class B extends A {
-  B(p, b) : super(p, b);
-}
-
-bar(v) {
-  for (var x = v; x != null; x = x.p) {
-    if (x.b()) {
-      return x;
-    }
-  }
-  return null;
-}
-
-foo(v) {
-  A x = bar(v);
-  return x != null;
-}
-
-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));
-}