unregisterMixin(mixinId) {
    var i;
    var mixinEls = this.mixinEls;
    var mixinEl;
    for (i = 0; i < mixinEls.length; ++i) {
      mixinEl = mixinEls[i];
      if (mixinId === mixinEl.id) {
        mixinEls.splice(i, 1);
        break;
      }
    }
  }
Removes the mixin element from `mixinEls`.

@param {string} mixinId - ID of the mixin to remove.