js/libs/ember-1.5.1.js
function(key, value) {
    var attributeName = this._unspecifiedAttributeBindings && this._unspecifiedAttributeBindings[key];
    if (attributeName) {
      this._setupAttributeBindingObservation(key, attributeName);
    }

    defineProperty(this, key);
    return set(this, key, value);
  }
function(key, value) {
    var attributeName = this._unspecifiedAttributeBindings && this._unspecifiedAttributeBindings[key];
    if (attributeName) {
      this._setupAttributeBindingObservation(key, attributeName);
    }

    defineProperty(this, key);
    return set(this, key, value);
  }
Gets defined after initialization by _applyAttributeBindings