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

Source 

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