this.isRTL 不是使用paper-dropdown-menu webcomponent时的函数错误

this.isRTL is not a function error when using paper-dropdown-menu webcomponent

加载 paper-dropdown-menu webcomponent 时出现此错误。

Uncaught TypeError: this._isRTL is not a function

更长的 SEO 堆栈跟踪：

Uncaught TypeError: this._isRTL is not a function get _localeHorizontalAlign @ iron-dropdown.html:306_updateOverlayPosition @ iron-dropdown.html:461_positionTargetChanged @ iron-dropdown.html:405_observerEffect @ polymer.html:1562_effectEffects @ polymer.html:1405_propertySetter @ polymer.html:1389setter @ polymer.html:1468attached @ iron-fit-behavior.html:222_invokeBehavior @ polymer-micro.html:433_doBehavior @ polymer-micro.html:426(anonymous function) @ polymer-micro.html:210_makeReady @ polymer-micro.html:111(anonymous function) @ polymer-micro.html:117 polymer-mini.html:2046 Uncaught TypeError: Cannot read property 'getWidth' of undefined(anonymous function) @ app-drawer-layout.html:189complete @ polymer-mini.html:2085Debouncer.boundComplete @ polymer-mini.html:2062_atEndOfMicrotask @ polymer-mini.html:2040window.MutationObserver.observe.characterData @ polymer-mini.html:2055

显然 Polymer 1.5 与 paper-dropdown-menu 版本 ~1.2.1 不兼容。我必须降级到 Polymer 1.4 才能使用 paper-dropdown-menu。

编辑：

问题是这个组件与影子不兼容 DOM。