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;
}
Check if any mixins contains a component.

@param {string} name - Component name.
@param {Array<Element>} mixinEls - Array of <a-mixin>s.