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

pkg/front_end/testcases/inference/for_loop_promotion.dart.dartk.expect

Index: pkg/front_end/testcases/inference/for_loop_promotion.dart.dartk.expect
diff --git a/pkg/front_end/testcases/inference/for_loop_promotion.dart.dartk.expect b/pkg/front_end/testcases/inference/for_loop_promotion.dart.dartk.expect
new file mode 100644
index 0000000000000000000000000000000000000000..7f01ee0e23cfae027bf8371fbfe657485fcadb5e
--- /dev/null
+++ b/pkg/front_end/testcases/inference/for_loop_promotion.dart.dartk.expect
@@ -0,0 +1,12 @@
+library test;
+import self as self;
+import "dart:core" as core;
+
+static method test() → void {
+  for (core::num x = 0; x.<(10); x = x.+(1)) {
+    if(x is core::int) {
+      dynamic y = x;
+    }
+  }
+}
+static method main() → dynamic {}