emberjs/ember.js — pull request #5513 by stefanpenner (2014-08-30 00:50:20+00:00)
Title: speedup watchKey by preventing for in related deopt

another tiny microperf

keyName in obj if obj is not really enumerable or a complex object causes the entire function to deopt. As MANDATORY_SETTER is only a development thing, in production we don’t want it’s guard deopting us.

Someday this whole block should be stripped for production builds