Improve the performance of closure-converted code.

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

 library;
 import self as self;
 import "dart:core" as core;
 
 static method main(core::List<core::String> arguments) → dynamic {
   final (dynamic) → dynamic foo = MakeClosure<(dynamic) → dynamic>(self::closure#main#foo, null);
   foo.call(arguments.[](0)).call();
 }
 static method closure#main#foo#bar(dynamic #contextParameter) → dynamic {
   core::print(#contextParameter[2]);
 }
 static method closure#main#foo(dynamic #contextParameter, dynamic x) → dynamic {
   final dynamic #context = MakeVector(3);
   #context[1] = #contextParameter;
   #context[2] = x;
+  x = null;
   final () → dynamic bar = MakeClosure<() → dynamic>(self::closure#main#foo#bar, #context);
   return bar;
 }