Improve the performance of closure-converted code.

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

 library;
 import self as self;
 import "dart:core" as core;
 
 class X extends core::Object {
   default constructor •() → void
     : super core::Object::•()
     ;
 }
 class A extends core::Object {
   field self::X x;
   constructor •(self::X x) → void
     : self::A::x = x, super core::Object::•() {}
 }
 class B extends core::Object {
   field self::X x;
   constructor •(self::X x) → void
     : final dynamic #context = MakeVector(3), this self::B::#redir(x, #context)
     ;
   constructor #redir(self::X x, final dynamic #context) → void
-    : dynamic #t1 = #context[2] = x, self::B::x = #context[2], super core::Object::•() {
+    : dynamic #t1 = #context[2] = x, dynamic #t2 = x = null, self::B::x = #context[2], super core::Object::•() {
     final () → dynamic fn = MakeClosure<() → dynamic>(self::closure#B#function#fn, #context);
     fn.call();
   }
 }
 static method main() → dynamic {
   self::A a = new self::A::•(new self::X::•());
   self::B b = new self::B::•(new self::X::•());
 }
 static method closure#B#function#fn(dynamic #contextParameter) → dynamic {
   core::print(#contextParameter[2]);
 }