closure conversion: Do not set Context.parent if the value is `null`

pkg/kernel/testcases/closures/uncaptured_for_in_loop.dart.expect

Index: pkg/kernel/testcases/closures/uncaptured_for_in_loop.dart.expect
diff --git a/pkg/kernel/testcases/closures/uncaptured_for_in_loop.dart.expect b/pkg/kernel/testcases/closures/uncaptured_for_in_loop.dart.expect
index eb0f413218f30b31cb80bbf929f8a4851a8838f1..c506df15f7f2bb3012a61d76c05ba775e9f7172f 100644
--- a/pkg/kernel/testcases/closures/uncaptured_for_in_loop.dart.expect
+++ b/pkg/kernel/testcases/closures/uncaptured_for_in_loop.dart.expect
@@ -20,7 +20,6 @@ static method main() → dynamic {
   dynamic closures = <dynamic>[];
   for (core::int i in self::numbers) {
     final mock::Context #context = new mock::Context::•(1);
-    #context.parent = null;
     #context.[]=(0, i);
     closures.add(new self::Closure#main#function::•(#context));
   }