Remove support for attribute event listeners.

client/dom/benchmarks/dromaeo/Dromaeo.dart

 #import('dart:dom');
 #import('dart:json');
 #import('Suites.dart');
 
 main() {
-   window.onload = (Event evt) {
-    new Dromaeo().run();
-  };
+  new Dromaeo().run();
 }
 
 class SuiteController {
   final SuiteDescription _suiteDescription;
   final HTMLIFrameElement _suiteIframe;
 
   HTMLDivElement _element;
   double _meanProduct;
   int _nTests;
 
@@ skipping 180 matching lines @@
   }
 
   HTMLElement _byId(String id) {
     return document.getElementById(id);
   }
 
   int get _suitesTotal() {
     return _suiteControllers.length;
   }
 }