Fix for issue 2468: also convert Dart closures to Js closures in stubs for native methods.

frog/tests/native/src/NativeWrappingFunction3FrogTest.dart

 // Copyright (c) 2012, 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.
 
 typedef void Callback0();
 typedef void Callback1(arg1);
 typedef void Callback2(arg1, arg2);
 
 class A native "*A" {
-  foo0(Callback0 closure) native;
-  foo1(Callback1 closure, arg1) native;
-  foo2(Callback2 closure, arg1, arg2) native;
+  foo1(Callback1 closure, [arg1 = 0]) native;
+  foo2(Callback2 closure, [arg1 = 0, arg2 = 1]) native;
 }
 
 makeA() native;
 
 void setup() native """
 function A() {}
-A.prototype.foo0 = function(closure) { return closure(); };
 A.prototype.foo1 = function(closure, arg1) { return closure(arg1); };
 A.prototype.foo2 = function(closure, arg1, arg2) {
   return closure(arg1, arg2);
 };
 makeA = function(){return new A;};
 """;
 
 
 main() {
   setup();
   var a = makeA();
-  Expect.equals(42, a.foo0(() => 42));
   Expect.equals(43, a.foo1((arg1) => arg1, 43));
+  Expect.equals(43, a.foo1((arg1) => arg1, arg1: 43));
+  Expect.equals(0, a.foo1((arg1) => arg1));
+
   Expect.equals(44, a.foo2((arg1, arg2) => arg1 + arg2, 21, 23));
+  Expect.equals(22, a.foo2((arg1, arg2) => arg1 + arg2, 21));
+  Expect.equals(22, a.foo2((arg1, arg2) => arg1 + arg2, arg1: 21));
+  Expect.equals(21, a.foo2((arg1, arg2) => arg1 + arg2, arg2: 21));
+  Expect.equals(44, a.foo2((arg1, arg2) => arg1 + arg2, arg1: 21, arg2: 23));
+  Expect.equals(44, a.foo2((arg1, arg2) => arg1 - arg2, arg2: 22, arg1: 66));
 
   A aa = a;
-  Expect.equals(42, aa.foo0(() => 42));
   Expect.equals(43, aa.foo1((arg1) => arg1, 43));
+  Expect.equals(43, aa.foo1((arg1) => arg1, arg1: 43));
+  Expect.equals(0, aa.foo1((arg1) => arg1));
+
   Expect.equals(44, aa.foo2((arg1, arg2) => arg1 + arg2, 21, 23));
+  Expect.equals(22, aa.foo2((arg1, arg2) => arg1 + arg2, 21));
+  Expect.equals(22, aa.foo2((arg1, arg2) => arg1 + arg2, arg1: 21));
+  Expect.equals(21, aa.foo2((arg1, arg2) => arg1 + arg2, arg2: 21));
+  Expect.equals(44, aa.foo2((arg1, arg2) => arg1 + arg2, arg1: 21, arg2: 23));
+  Expect.equals(44, aa.foo2((arg1, arg2) => arg1 - arg2, arg2: 22, arg1: 66));
 }