Provide class for stopping writing to undeclared properties
Summary:
This generalizes D3607 and future D3601.
It should be trait since PHP 5.4.
Test Plan: Will use it.
Reviewers: epriestley
Reviewed By: epriestley
CC: aran, Korvin, btrahan
Differential Revision: https://secure.phabricator.com/D3618