Improve the performance of closure-converted code.

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

 library;
 import self as self;
 import "dart:core" as core;
 
 typedef fn = (dynamic) → dynamic;
 typedef fn2 = (dynamic, dynamic) → dynamic;
 class X extends core::Object {
   default constructor •() → void
     : super core::Object::•()
     ;
 }
 static method startIsolateMock(dynamic parentPort, dynamic entryPoint, dynamic args, dynamic message, dynamic isSpawnUri, dynamic controlPort, core::List<dynamic> capabilities) → void {
   final dynamic #context = MakeVector(7);
   #context[2] = entryPoint;
+  entryPoint = null;
   #context[3] = args;
+  args = null;
   #context[4] = message;
+  message = null;
   #context[5] = isSpawnUri;
+  isSpawnUri = null;
   if(!controlPort.==(null)) {
     controlPort.handler = MakeClosure<(dynamic) → dynamic>(self::closure#startIsolateMock#function, #context);
   }
   if(!parentPort.==(null)) {
     dynamic readyMessage = core::List::_internal<dynamic>(2);
     readyMessage.[]=(0, controlPort.sendPort);
     readyMessage.[]=(1, capabilities);
     capabilities = null;
     parentPort.send(readyMessage);
   }
@@ skipping 15 matching lines @@
         (#contextParameter[2]).call(#contextParameter[3]);
       }
       else {
         (#contextParameter[2]).call();
       }
   }
   else {
     (#contextParameter[2]).call(#contextParameter[4]);
   }
 }