code: function isComponentMixedIn (name, mixinEls) {
    var i;
    var inMixin = false;
    for (i = 0; i < mixinEls.length; ++i) {
      inMixin = mixinEls[i].hasAttribute(name);
      if (inMixin) { break; }
    }
    return inMixin;
  }
docs: Check if any mixins contains a component.
  
  @param {string} name - Component name.
  @param {array} mixinEls - Array of <a-mixin>s.