Improve the performance of closure-converted code.

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

 library;
 import self as self;
 import "dart:core" as core;
 
 static method range(core::int high) → dynamic {
   final dynamic #context = MakeVector(3);
   #context[2] = high;
+  high = null;
   final (core::int) → dynamic iter = MakeClosure<(core::int) → dynamic>(self::closure#range#iter, #context);
   return iter;
 }
 static method main() → dynamic {
   dynamic sum = 0;
   for (dynamic x in self::range(10).call(2))
     sum = sum.+(x);
   if(!sum.==(44)) {
     throw core::Exception::•("Incorrect output.");
   }
 }
 static method closure#range#iter(dynamic #contextParameter, core::int low) → dynamic /* originally sync* */ {
   final dynamic #context = MakeVector(3);
   #context[1] = #contextParameter;
   #context[2] = low;
+  low = null;
   dynamic :await_jump_var = 0;
   dynamic :await_ctx_var;
   dynamic :sync_op = (core::Iterator<dynamic> :iterator) → core::bool yielding {
     {
       while ((let final dynamic #t1 = #contextParameter[2] in let final dynamic #t2 = #contextParameter[2] = #t1.-(1) in #t1).>(#context[2])) {
         :iterator._current = #context[1][2];
         [yield] true;
       }
     }
     return false;
   };
   return new core::_SyncIterable::•(:sync_op);
 }