getDOMAttribute(attr) {
    // If cached value exists, return partial component data.
    var component = this.components[attr];
    if (component) {
      return component.attrValue;
    }
    return window.HTMLElement.prototype.getAttribute.call(this, attr);
  }
If `attr` is a component, returns JUST the component data defined on the entity.
Like a partial version of `getComputedAttribute` as returned component data
does not include applied mixins or defaults.

If `attr` is not a component, fall back to HTML getAttribute.

@param {string} attr
@returns {object|string} Object if component, else string.