Entity field access condition 2281113

This is a first pass on #2281113.  Plugin is implemented and the test runs green, but only hits the "allowed" case.  Other questions:

* How should language be handled?  I'm taking the "default language", and I'm not sure that's right.
* I don't test the "not allowed" case.  I'd guess that I should modify the test module, and make SimpleTest include it.  Is this what I should do?
* I need to check coding standards, since it looks like we do this differently in 8.x.
* I worked to get this working;  it probably needs to be changed to correctly support DI.