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

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

 library;
 import self as self;
 import "dart:core" as core;
 import "dart:mock" as mock;
 
 class Closure#main#function extends core::Object implements core::Function {
   field core::String note = "This is temporary. The VM doesn't need closure classes.";
   field mock::Context context;
   constructor •(final mock::Context context) → dynamic
     : self::Closure#main#function::context = context
     ;
   method call() → dynamic {
     "This is a temporary solution. In the VM, this will become an additional parameter.";
     final mock::Context #contextParameter = this.{self::Closure#main#function::context};
     return #contextParameter.[](0);
   }
 }
 static const field dynamic numbers = const <core::int>[0, 1, 2, 3, 4, 5, 6, 7, 8, 9];
 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));
     }
   }
   core::int sum = 0;
   for (core::Function f in closures) {
     sum = sum.+(f.call());
   }
   core::int expectedSum = self::numbers.length.-(1).*(self::numbers.length).~/(2);
   if(!expectedSum.==(sum)) {
     throw core::Exception::•("Unexpected sum = ${sum} != ${expectedSum}");
   }
 }