title: RequiredFieldValidator not firing when ControlToValidate visiblity is changed
body: <p>When I change ddl.Visible=true(it gets changed on partial postback/updatepanel from another form element event)the RequiredFieldValidator will not fire?</p>
  
  <p>NOTE: This is not a question on how to use the RequiredFieldValidator in a normal circumstance.  My form has cascading dropdowns that are all dynamically built with their visibility toggled on and off.</p>
  
  <pre><code>&lt;asp:DropDownList ID="ddl" Visible="false" AutoPostBack="True" runat="server"&gt;                                                               
  &lt;/asp:DropDownList&gt;
  &lt;asp:RequiredFieldValidator ControlToValidate="ddl" 
  ID="RequiredFieldValidator1" 
  runat="server" ErrorMessage="Required"&gt;&lt;/asp:RequiredFieldValidator&gt;       
  </code></pre>