Canonicalize closures passed to native functions.

frog/leg/lib/js_helper.dart

Index: frog/leg/lib/js_helper.dart
===================================================================
--- frog/leg/lib/js_helper.dart	(revision 5170)
+++ frog/leg/lib/js_helper.dart	(working copy)
@@ -1333,7 +1333,10 @@
  * closure when the Dart closure is passed to the DOM.
  */
 convertDartClosureToJS(closure) {
-  return JS("var", @"""function() {
+  if (JS('bool', @'"$identity" in $0', closure)) {

[kasperl, 2012/03/08 15:06:08]

The 'in' construct is probably not going to perform very well. How about
something like this:

   var function = JS('var', @'$0.$identity', closure);
   if (JS('bool', @'!!$0', function)) return function;

[ngeoffray, 2012/03/08 15:11:36]

OK, I copied the test from line 1276, I'll change it there too (unfortunately
the code cannot be shared because of how we analyze JS).

[ngeoffray, 2012/03/08 15:24:01]

There is actually a good reason why the code line 1276 uses 'in'. The exception
thrown may have been null. I have added a comment there.

+    return JS('var', @'$0.$identity', closure);
+  }
+  var function = JS("var", @"""function() {
     var dartClosure = $0;
     switch (arguments.length) {
       case 0: return $1(dartClosure);
@@ -1347,6 +1350,8 @@
   callClosure0,
   callClosure1,
   callClosure2);
+  JS('void', @'$0.$identity = $1', closure, function);
+  return function;
 }
 
 /**