Improve the performance of closure-converted code.

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

 library;
 import self as self;
 import "dart:core" as core;
 
 static field dynamic f;
 static method foo() → dynamic {
   core::print(self::f.call(0));
 }
 static method main(dynamic arguments) → dynamic {
   final dynamic #context = MakeVector(3);
   #context[2] = arguments;
+  arguments = null;
   final (dynamic) → dynamic g = MakeClosure<(dynamic) → dynamic>(self::closure#main#g, #context);
   self::f = g;
   self::foo();
 }
 static method closure#main#g(dynamic #contextParameter, dynamic x) → dynamic {
   return (#contextParameter[2]).[](x);
 }