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

dart/lib/compiler/implementation/typechecker.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.
 
 class TypeCheckerTask extends CompilerTask {
   TypeCheckerTask(Compiler compiler) : super(compiler);
   String get name() => "Type checker";
 
   static final bool LOG_FAILURES = false;
 
@@ skipping 87 matching lines @@
   toString() {
     StringBuffer sb = new StringBuffer();
     bool first = true;
     sb.add('(');
     parameterTypes.printOn(sb, ', ');
     sb.add(') -> ${returnType}');
     return sb.toString();
   }
 
   SourceString get name() => const SourceString('Function');
+
+  int computeArity() {
+    int arity = 0;
+    parameterTypes.forEach((_) { arity++; });
+    return arity;
+  }
 }
 
 class Types {
   static final VOID = const SourceString('void');
   static final INT = const SourceString('int');
   static final DOUBLE = const SourceString('double');
   static final DYNAMIC = const SourceString('Dynamic');
   static final STRING = const SourceString('String');
   static final BOOL = const SourceString('bool');
   static final OBJECT = const SourceString('Object');
@@ skipping 656 matching lines @@
   }
 
   visitCatchBlock(CatchBlock node) {
     return unhandledStatement();
   }
 
   visitTypedef(Typedef node) {
     return unhandledStatement();
   }
 }