Protractor: Find Element by Attribute

I have the following element i need to find for testing:

<div class="alert alert-danger" role="alert" ng-show="notValid">Zugangsdaten eingeben</div>

How can i find this element to check visibility (ng-show)?

The ng-show attribute and value are the only attribute and value to identify the element uniquely. The class is used in many elements...

I am searching for something like:

var notValid = element(by.Attribute('ng-show', 'notValid');