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

pkg/kernel/testcases/closures/for_variable_capture_test.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 method main() → dynamic {
   dynamic closure;
   {
     mock::Context #context = new mock::Context::•(1);
-    #context.parent = null;
     #context.[]=(0, 0);
     dynamic fn = new self::Closure#main#function::•(#context);
     for (; #context.[](0).<(3); #context = #context.copy(), #context.[]=(0, #context.[](0).+(1))) {
       #context.[]=(0, #context.[](0).+(1));
       closure = fn;
     }
   }
   dynamic x = closure.call();
   if(!x.==(1)) {
     throw "Expected 1, but got ${x}.";
   }
 }