Do not convert a Dart closure parameter to a JS closure if the parameter was not provided.

frog/tests/frog_native/native_wrapping_function2_frog_test.dart

Index: frog/tests/frog_native/native_wrapping_function2_frog_test.dart
===================================================================
--- frog/tests/frog_native/native_wrapping_function2_frog_test.dart	(revision 8546)
+++ frog/tests/frog_native/native_wrapping_function2_frog_test.dart	(working copy)
@@ -10,6 +10,8 @@
   foo0(Callback0 closure) native "return closure();";
   foo1(Callback1 closure, arg1) native "return closure(arg1);";
   foo2(Callback2 closure, arg1, arg2) native "return closure(arg1, arg2);";
+  foo3([Callback0 closure]) native
+      "return closure == (void 0) ? 42 : closure();";
 }
 
 makeA() native;
@@ -26,9 +28,13 @@
   Expect.equals(42, a.foo0(() => 42));
   Expect.equals(43, a.foo1((arg1) => arg1, 43));
   Expect.equals(44, a.foo2((arg1, arg2) => arg1 + arg2, 21, 23));
+  Expect.equals(42, a.foo3());
+  Expect.equals(43, a.foo3(() => 43));
 
   A aa = a;
   Expect.equals(42, aa.foo0(() => 42));
   Expect.equals(43, aa.foo1((arg1) => arg1, 43));
   Expect.equals(44, aa.foo2((arg1, arg2) => arg1 + arg2, 21, 23));
+  Expect.equals(42, aa.foo3());
+  Expect.equals(43, aa.foo3(() => 43));
 }