Migrated test block 16 to Dart 2.0.

tests/corelib_strong/map_keys2_test.dart

-// Copyright (c) 2011, 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";
-
-main() {
-  var map1 = {"foo": 42, "bar": 499};
-  var map2 = {};
-  var map3 = const {"foo": 42, "bar": 499};
-  var map4 = const {};
-  var map5 = new Map<String, int>();
-  map5["foo"] = 43;
-  map5["bar"] = 500;
-  var map6 = new Map<String, bool>();
-
-  Expect.isTrue(map1.keys is Iterable<String>);
-  Expect.isTrue(map1.keys is Iterable<bool>);
-
-  Expect.isTrue(map2.keys is Iterable<String>);
-  Expect.isTrue(map2.keys is Iterable<bool>);
-
-  Expect.isTrue(map3.keys is Iterable<String>);
-  Expect.isTrue(map3.keys is Iterable<bool>);
-
-  Expect.isTrue(map4.keys is Iterable<String>);
-  Expect.isTrue(map4.keys is Iterable<bool>);
-
-  Expect.isTrue(map5.keys is Iterable<String>);
-  Expect.isFalse(map5.keys is Iterable<bool>);
-
-  Expect.isTrue(map6.keys is Iterable<String>);
-  Expect.isFalse(map6.keys is Iterable<bool>);
-}