Reverts: flutter/flutter#143176
Initiated by: QuncCccccc
Reason for reverting: made tree red.
Original PR Author: Hixie

Reviewed By: {Piinks}

This change reverts the following previous change:
This is part 4 of a broken down version of the #140101 refactor.

This PR renames isAvailableForEnvironment to isForEnvironment and replaces a regular expression with a simple function. (The latter will change the behaviour for people with branch names like `mainly_refactors` or `chess_master_experiment` or whatever, but I'm pretty sure the old behaviour was not intended.)