Migrate block 162 to Dart 2.0.

tests/language_2/type_propagation2_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.
 
 // Regression test for dart2js that used to infinite loop on
 // speculatively propagating types.
 
 class Bar {
   noSuchMethod(e) => null;
 }
 
 main() {
   var d = new Bar();
 
   while (false) {
     // [input] will change from indexable to unknown: the use line 20
     // changes its decision because [a2] changes its type from unknown to
     // null.
     var input = ((x) {})(null);
     var p2 = input.keys.firstWhere(null);
     var a2 = input.keys.firstWhere(null);
     print(input[a2] == p2);
   }
 }