Migrate block 162 to Dart 2.0.

tests/language_strong/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);
-  }
-}