Fix implementation of Function.apply with optional positional arguments.

tests/corelib/apply4_test.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import "package:expect/expect.dart";
 
 // Testing Function.apply calls work correctly for arities that are not
 // otherwise present in the program (and thus might not have stubs
 // generated).
 
 class A {
   foo(x, [y, z, a, b, c, d = 99, e, f, g, h, i, j]) => "$x $d";
 }
 
 main() {
   var a = new A();
   var clos = a.foo;
   Expect.equals(Function.apply(clos, ["well"]), "well 99");
-  Expect.equals(Function.apply(clos, ["well", 0, 1, 2, 3, 4, 5, 6]), "well 5");
+  Expect.equals(Function.apply(clos, ["well", 0, 2, 4, 3, 6, 9, 10]), "well 9");
 }