Support positional optional parameters even if stubs are missing.

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");
+}