Elemental2 Error

I have been using GWT for a while but I am new to elemental2, I have tried the following code to create new navigator object in GWT:

import elemental2.dom.Navigator;

and for entrypoint

private Navigator Nav = new Navigator(); 

the following are the first several lines of the error message I got during compile:

 

            [ERROR] elemental2.dom.Navigator cannot be resolved to a type

         [ERROR] Errors in 'elemental2/dom/Navigator.java'

            [ERROR] elemental2.dom.ServiceWorkerContainer cannot be resolved to a type

         [ERROR] Errors in 'elemental2/dom/BatteryManager.java'

            [ERROR] elemental2.dom.EventTarget cannot be resolved to a type

can anyone help? 

Thanks,

Joe

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