Method sync

src/polyfills/custom-elements.js:491–495

* This causes a synchronous handling of all the Mutation Observer's tracked * mutations. This does nothing until the mutation observer is actually * registered on the first Custom Element definition.

()

   * registered on the first Custom Element definition.
   */
  sync() {
    if (this.mutationObserver_) {
      this.handleRecords_(this.mutationObserver_.takeRecords());
    }
  }

  /**
   * Handle all the Mutation Observer's Mutation Records.