In non-strong mode, don't infer the type of for-in loop variables.

pkg/front_end/testcases/inference/for_in_loop_promotion.dart.direct.expect

Index: pkg/front_end/testcases/inference/for_in_loop_promotion.dart.direct.expect
diff --git a/pkg/front_end/testcases/inference/for_in_loop_promotion.dart.direct.expect b/pkg/front_end/testcases/inference/for_in_loop_promotion.dart.direct.expect
index dcff01acbeac3f6a38f8c415199a57eca6990cfc..e0d10e2d0e1a3ce5a9af8d0f1409f68bc5c6276b 100644
--- a/pkg/front_end/testcases/inference/for_in_loop_promotion.dart.direct.expect
+++ b/pkg/front_end/testcases/inference/for_in_loop_promotion.dart.direct.expect
@@ -3,7 +3,7 @@ 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) {
+  for (dynamic x in nums) {
     if(x is core::int) {
       dynamic y = x{core::int};
     }