[BUGFIX release] make GUID_KEY = intern(GUID_KEY) actually work on ES3.

Ember.keys() filters out __ leading underscores so it returns undefined for GUID_KEY. This change not only works in ES3 browsers but is faster across all browsers I benchmarked.
Merge pull request #5612 from emberjs/fix-intern

[BUGFIX release] make GUID_KEY = intern(GUID_KEY) actually work on ES3.