Stop passing beforeChange as token to aRCL API

This triggered issues downstream where the second argument is expected
to be the token, but was instead the before flag.

I don't have a specific test case for this within FRB itself, so it might
warrant further investigation to lock in the desired functionality.

I couldn't find a token worth passing along so I went with null just to
match the expected method signature.

This addresses the failing spec on Montage when applying a fix in the
property change listening on bound properties: montagejs/montage#1334