Use the typedef arity to know how to invoke a closure given by the dom.

dart/lib/compiler/implementation/typechecker.dart

Index: dart/lib/compiler/implementation/typechecker.dart
===================================================================
--- dart/lib/compiler/implementation/typechecker.dart	(revision 8049)
+++ dart/lib/compiler/implementation/typechecker.dart	(working copy)
@@ -105,6 +105,12 @@
   }
 
   SourceString get name() => const SourceString('Function');
+
+  int arity() {

[ahe, 2012/05/29 12:32:08]

This method does not have the performance characteristics that you would
naturally expect. How about calling it computeArity?

[ngeoffray, 2012/05/29 13:06:28]

Done.

+    int arity = 0;
+    parameterTypes.forEach((_) { arity++; });
+    return arity;
+  }
 }
 
 class Types {