Dynamically create polymer element

I have defined login element as follows:

<polymer-element name="my-login" extends="form">
  <template> ...</template>

and dart class as follows:

@CustomTag('my-login')
  class MyLogin extends PolymerElement { .... }

In the main application component, I am trying to create my-login as follows:

 MyLogin p =  new Element.tag("my-login", "FormElement");

This generates exception:

Exception: type 'HtmlElement' is not a subtype of type 'MyLogin' of 'p'.

The code works if "extends=form" is removed from polymer-element declaration :

 <polymer-element name="my-login">

and created as follows:

MyLogin p =  new Element.tag("my-login");

How to dynamically create custom element that extends other HTML element?