[BUG][CANARY] Mixins throwing `object is not extensible`

[kiwiupover]

Both addon are throwing TypeError: Cannot add property undefined, object is not extensible
https://github.com/offirgolan/ember-cp-validations/blob/master/addon/validations/factory.js#L146
https://github.com/offirgolan/ember-parachute/blob/master/addon/query-params.js#L293

ValidationsMixin[Ember.NAME_KEY] = 'Validations';
Ember.NAME_KEY is undefined
ValidationsMixin seems to be immutable

Addons using Ember.NAME_KEY
https://emberobserver.com/code-search?codeQuery=NAME_KEY&sort=score&sortAscending=false

[rwjblue]

Steps to fix:

1. Add this[NAME_KEY] = null; to the Mixin constructor before calling Object.seal(this).
2. Add a deprecation when accessing Ember.NAME_KEY (suggesting the usage of toString directly).
3. Profit? 💸