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

pkg/kernel/testcases/closures/for_loop.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
@@ skipping 15 matching lines @@
     final mock::Context #contextParameter = this.{self::Closure#main#function#1::context};
     if(!#contextParameter.parent.[](0).==(self::max.-(1)))
       throw "last: ${#contextParameter.parent.[](0)} != ${self::max.-(1)}";
   }
 }
 static const field core::int max = 100;
 static method main() → dynamic {
   dynamic closures = <dynamic>[];
   dynamic closures2 = <dynamic>[];
   final mock::Context #context = new mock::Context::•(1);
-  #context.parent = null;
   #context.[]=(0, null);
   {
     mock::Context #context = new mock::Context::•(1);
     #context.parent = #context;
     #context.[]=(0, 0);
     for (; #context.[](0).<(self::max); #context = #context.copy(), #context.[]=(0, #context.[](0).+(1))) {
       closures.add(new self::Closure#main#function::•(#context));
       closures2.add(new self::Closure#main#function#1::•(#context));
     }
   }
   core::int sum = 0;
   for (core::Function f in closures) {
     sum = sum.+(f.call());
   }
   for (core::Function f in closures2) {
     f.call();
   }
   core::int expectedSum = self::max.-(1).*(self::max).~/(2);
   if(!expectedSum.==(sum)) {
     throw core::Exception::•("Unexpected sum = ${sum} != ${expectedSum}");
   }
 }