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.direct.expect

+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) {
+      dynamic y = x{core::int};
+    }
+  }
+}
+static method main() → dynamic {}