registerMixin(mixinId) {
    var compositedMixinIds;
    var i;
    var mixin;
    var mixinEl = document.getElementById(mixinId);
    if (!mixinEl) {
      warn('No mixin was found with id `%s`', mixinId);
      return;
    }

    // Register composited mixins (if mixin has mixins).
    mixin = mixinEl.getAttribute('mixin');
    if (mixin) {
      compositedMixinIds = _utils_index_js__WEBPACK_IMPORTED_MODULE_0__.split(mixin.trim(), /\s+/);
      for (i = 0; i < compositedMixinIds.length; i++) {
        this.registerMixin(compositedMixinIds[i]);
      }
    }

    // Register mixin.
    this.computedMixinStr = this.computedMixinStr + ' ' + mixinEl.id;
    this.mixinEls.push(mixinEl);
  }
From mixin ID, add mixin element to `mixinEls`.

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