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

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

Index: pkg/kernel/testcases/closures/capture_this.dart.expect
diff --git a/pkg/kernel/testcases/closures/capture_this.dart.expect b/pkg/kernel/testcases/closures/capture_this.dart.expect
index 442f00a28931f5c45badf402658f2ec386b8892d..12e9721b1cf8a8ec6a0766ab961982f651fd6e15 100644
--- a/pkg/kernel/testcases/closures/capture_this.dart.expect
+++ b/pkg/kernel/testcases/closures/capture_this.dart.expect
@@ -10,13 +10,11 @@ class C extends core::Object {
     ;
   method m() → dynamic {
     final mock::Context #context = new mock::Context::•(1);
-    #context.parent = null;
     #context.[]=(0, this);
     return new self::Closure#C#m#function::•(#context);
   }
   method f() → dynamic {
     final mock::Context #context = new mock::Context::•(1);
-    #context.parent = null;
     #context.[]=(0, this);
     return new self::Closure#C#f#function::•(#context);
   }