prototype: Fix $F breakage from [6598].

CHANGELOG

@@ -1,5 +1,7 @@
 *SVN*
 
+* Fix $F breakage from [6598].  [sam]
+
 * Set window.$continue to an Error with a helpful message for anyone who may still be using "throw $continue".  [sam]
 
 * Fix jstest.rb IE support so you don't have to close the IE window after each test (yay!).  Closes #8207.  [Ryan Schuft]

test/unit/form.html

@@ -122,6 +122,10 @@
       $('tf_radio').checked = false;
     }},
     
+    testDollarF: function(){ with(this) {
+      assertEqual("4", $F("input_enabled"));
+    }},
+    
     testFormElementEventObserver: function(){ with(this) {
       var observer = new Form.Element.EventObserver('input_enabled', function(){
         callbackCounter++;