Add v3.12.2 to CHANGELOG.md.

[ci skip]

Author: Robert Jackson

CHANGELOG.md

@@ -87,6 +87,10 @@
 - [#18217](https://github.com/emberjs/ember.js/pull/18217) [BUGFIX] Adds ability for computed props to depend on args
 - [#18222](https://github.com/emberjs/ember.js/pull/18222) [BUGFIX] Matches assertion behavior for CPs computing after destroy
 
+### v3.12.2 (December 4, 2019)
+
+- [#18576](https://github.com/emberjs/ember.js/pull/18576) [BUGFIX] Fix leftover `const` expressions in legacy builds
+
 ### v3.12.1 (November 20, 2019)
 
 - [#18244](https://github.com/emberjs/ember.js/pull/18244) Fix query param assertion when using the router services `transitionTo` to redirect _during_ an existing transition.