1. DomName('DocumentFragment.getElementById')
  2. DocsEditable()
  3. Experimental()
Element getElementById(String elementId)

Source 

@DomName('DocumentFragment.getElementById')
@DocsEditable()
@Experimental() // untriaged
Element getElementById(String elementId) => wrap_jso(_blink.BlinkDocumentFragment.instance.getElementById_Callback_1_(unwrap_jso(this), elementId));