Add a test to verify that variables declared in for-loops can be type promoted.

pkg/front_end/testcases/inference/for_loop_promotion.dart

+// Copyright (c) 2017, 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.
+
+/*@testedFeatures=inference*/
+library test;
+
+void test() {
+  for (num x = 0; x /*@target=num::<*/ < 10; x++) {
+    if (x is int) {
+      var /*@type=int*/ y = /*@promotedType=int*/ x;
+    }
+  }
+}
+
+main() {}