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

pkg/front_end/testcases/inference/for_in_loop_promotion.dart

Index: pkg/front_end/testcases/inference/for_in_loop_promotion.dart
diff --git a/pkg/front_end/testcases/inference/for_in_loop_promotion.dart b/pkg/front_end/testcases/inference/for_in_loop_promotion.dart
new file mode 100644
index 0000000000000000000000000000000000000000..5addd48c057508568c272b8388771e7244c285c5
--- /dev/null
+++ b/pkg/front_end/testcases/inference/for_in_loop_promotion.dart
@@ -0,0 +1,16 @@
+// 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;
+
+viod test(List<num> nums) {
+  for (var /*@type=num*/ x in nums) {
+    if (x is int) {
+      var /*@type=int*/ y = /*@promotedType=int*/ x;
+    }
+  }
+}
+
+main() {}