Add dojo as a dev-dependency because it's used by the test for Destroyable.
Make decor/Evented module to replace dojo/Evented.

There are a lot of ways to make this module.  I copied the technique from
dojo, but we could alternately:

1. emit events on the document itself, and then use
   addEventListener()/removeEventListener() rather than dcl.advise()
2. hand craft the code to keep track of who is listening for what event