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.strong.expect

Index: pkg/front_end/testcases/inference/for_in_loop_promotion.dart.strong.expect
diff --git a/pkg/front_end/testcases/inference/for_in_loop_promotion.dart.strong.expect b/pkg/front_end/testcases/inference/for_in_loop_promotion.dart.strong.expect
new file mode 100644
index 0000000000000000000000000000000000000000..bcff01673d5174b1ae245664332cf2b9b4302a68
--- /dev/null
+++ b/pkg/front_end/testcases/inference/for_in_loop_promotion.dart.strong.expect
@@ -0,0 +1,12 @@
+library test;
+import self as self;
+import "dart:core" as core;
+
+static method test(core::List<core::num> nums) → invalid-type {
+  for (core::num x in nums) {
+    if(x is core::int) {
+      core::int y = x{core::int};
+    }
+  }
+}
+static method main() → dynamic {}