Support for "a is Function".

frog/leg/lib/js_helper.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.
 
 #library('js_helper');
 
 #import('coreimpl.dart');
 
 #source('constant_map.dart');
 #source('date_helper.dart');
@@ skipping 1298 matching lines @@
 }
 
 /**
  * Called by generated code to fetch the stack trace from an
  * exception.
  */
 StackTrace getTraceFromException(exception) {
   return new StackTrace(JS("var", @"$0.stack", exception));
 }
 
+class StackTrace {
+  var stack;
+  StackTrace(this.stack);
+  String toString() => stack != null ? stack : '';
+}
+
+
 /**
  * Called by generated code to build a map literal. [keyValuePairs] is
  * a list of key, value, key, value, ..., etc.
  */
 makeLiteralMap(List keyValuePairs) {
   Iterator iterator = keyValuePairs.iterator();
   Map result = new LinkedHashMap();
   while (iterator.hasNext()) {
     String key = iterator.next();
     var value = iterator.next();
@@ skipping 28 matching lines @@
   return function;
 }
 
 /**
  * Helper methods when converting a Dart closure to a JS closure.
  */
 callClosure0(closure) => closure();
 callClosure1(closure, arg1) => closure(arg1);
 callClosure2(closure, arg1, arg2) => closure(arg1, arg2);
 
-class StackTrace {
-  var stack;
-  StackTrace(this.stack);
-  String toString() => stack != null ? stack : '';
+/**
+ * Super class for Dart closures.
+ */
+class Closure implements Function {
+  String toString() => "Closure";
 }
 
 bool jsHasOwnProperty(var jsObject, String property) {
   return JS('bool', @'$0.hasOwnProperty($1)', jsObject, property);
 }
 
 jsPropertyAccess(var jsObject, String property) {
   return JS('var', @'$0[$1]', jsObject, property);
 }