title: AjaxControlToolkit.TabPanel.prototype._header_onclick throws error
body: <p>AjaxControlToolkit.TabPanel.prototype._header_onclick  throws an error after upgrading ajaxtoolkit 1.0 to 3.0. </p>
  
  <p>Below is the code </p>
  
  <pre><code> AjaxControlToolkit.TabPanel.prototype._header_onclick = function(e) {
              this.raiseClick();
              if (isValidTabChange()) // add this additional code line to do validation
                  this.get_owner().set_activeTab(this);
          };
  </code></pre>
  
  <p>The error is AjaxControlToolkit is undefined and the code throws and error. Did anyone come across this issue? I might be doing something wrong, I want to know someone has resolved this issue before. </p>