1. @DomName('Element.attachShadow')
  2. @DocsEditable()
  3. @Experimental()
ShadowRoot attachShadow(Map shadowRootInitDict)

Source 

@DomName('Element.attachShadow')
@DocsEditable()
@Experimental() // untriaged
ShadowRoot attachShadow(Map shadowRootInitDict) =>
    _blink.BlinkElement.instance.attachShadow_Callback_1_(
        this, convertDartToNative_Dictionary(shadowRootInitDict));