Improve the performance of closure-converted code.

pkg/kernel/testcases/closures/loop2.dart.expect

 library;
 import self as self;
 import "dart:core" as core;
 
 static method doit(core::int x) → void {
   final dynamic #context = MakeVector(3);
   #context[2] = x;
+  x = null;
   final core::int max = 10;
   final core::double expectedSum = max.-(1).*(max)./(2);
   core::int counter = 0;
   dynamic calls = <dynamic>[];
   while (counter.<(max)) {
     final dynamic #context = MakeVector(3);
     #context[1] = #context;
     #context[2] = counter;
     calls.add(MakeClosure<() → dynamic>(self::closure#doit#function, #context));
     counter = counter.+(1);
   }
   core::double sum = 0.0;
   for (dynamic c in calls)
     sum = sum.+(c.call());
   if(!sum.==(expectedSum))
     throw core::Exception::•("Unexpected sum = ${sum} != ${expectedSum}");
 }
 static method main() → void {
   self::doit(0);
 }
 static method closure#doit#function(dynamic #contextParameter) → dynamic {
   return (#contextParameter[2]).+(#contextParameter[1][2]);
 }