code: function() {
      return this.__all__.reduce(function(all, mixin) {
        return mixin instanceof Mixin.__class__ ?
          all.concat(mixin.__flatten__()) :
          all.concat([mixin]);
      }, []);
    }
docs: Create a flattened array of properties are required to be mixed into a
  class in the order in which they should be mixed in.
  
  @method
  @private
  @return {Array.<Object>} The properties