code: public function hasMixin($mixinId)
      {
          return isset($this->mixins[$mixinId]) || isset($this->mixinsByCurie[$mixinId]);
      }
docs: Returns true if the mixin is on this schema.  Id provided can be
  qualified to major rev or just the curie.
  @see SchemaId::getCurieMajor
  
  @param string $mixinId
  
  @return bool