angular.equals needs to test for double $

[joshbowdoin]

Due to #6253, angular.equals needs to test for key.charAt(0) === '$' && key.charAt(1) === '$', not simply key.charAt(0) === '$'

The current test for single '$' breaks angular.equals, as well as anything that relies on it ($watch, for example).

[btford]

Can you please provide a reproduction via jsbin, plnkr.co or jsfiddle that shows how this change concretely affects you?

[joshbowdoin]

Use case - where checking a model for changes before persisting to server:
http://plnkr.co/edit/SVd3xVhhtFZ6XdOnaS8z?p=preview