I get this whenever I'm using VSCode and I add something similar to a component:

@Output() somename = new EventEmitter();
...and I let VSCode auto-import the supporting module. Instead of adding EventEmitter to the existing @angular/core, VSCode adds a new import to protractor which causes the error.

If you are getting this error and can't remember where the last changes were made, try searching for from 'protractor' in your code;