Remove support for attribute event listeners.

client/dom/benchmarks/dromaeo/tests/dom-attr.dart

 #library("dom-attr.dart");
 #import("dart:dom");
 #import('runner.dart');
 
 void main() {
   final int num = 10240;
 
   // Try to force real results.
   var ret;
-  window.onload = (Event evt) {
-    HTMLElement elem = document.getElementById('test1');
-    HTMLElement a = document.getElementsByTagName('a')[0];
 
-    new Suite(window, 'dom-attr')
-      .test('getAttribute', void _() {
+  HTMLElement elem = document.getElementById('test1');
+  HTMLElement a = document.getElementsByTagName('a')[0];
+
+  new Suite(window, 'dom-attr')
+    .test('getAttribute', void _() {
+      for (int i = 0; i < num; i++)
+        ret = elem.getAttribute('id');
+    })
+    .test('element.property', () {
+      for (int i = 0; i < num * 2; i++)
+        ret = elem.id;
+    })
+    .test('setAttribute', () {
         for (int i = 0; i < num; i++)
-          ret = elem.getAttribute('id');
-      })
-      .test('element.property', () {
-        for (int i = 0; i < num * 2; i++)
-          ret = elem.id;
-      })
-      .test('setAttribute', () {
-          for (int i = 0; i < num; i++)
-          a.setAttribute('id', 'foo');
-      })
-      .test('element.property = value', () {
-        for (int i = 0; i < num; i++)
-          a.id = 'foo';
-      })
-      .end();
-  };
+        a.setAttribute('id', 'foo');
+    })
+    .test('element.property = value', () {
+      for (int i = 0; i < num; i++)
+        a.id = 'foo';
+    })
+    .end();
 }