如何找到与Selenium一起使用的Polymer 2元素？

Question

我已经在本网站上查看了一些关于聚合物元素位置的答案，但仍然无法找到解决此阻塞问题的方法。

这是登录页面的Html源：

<paper-input no-label-float="" id="username" label="Username" autocomplete="on" required="" class="form-control style-scope colt-login x-scope paper-input-1" pattern="[A-Za-z0-9._%+-]+@[A-za-z0-9.-]+\.[A-Za-z]{2,10}$" tabindex="0" aria-disabled="false">
   <paper-input-container id="container" class="style-scope paper-input x-scope paper-input-container-1" dir="null">
      <div class="floated-label-placeholder style-scope paper-input-container" aria-hidden="true" hidden="">&nbsp;</div>
      <div class="input-wrapper style-scope paper-input-container">
         <span class="prefix style-scope paper-input-container"></span>
         <div id="labelAndInputContainer" class="input-content style-scope paper-input-container">
            <label aria-hidden="true" for="input" slot="label" class="style-scope paper-input" id="paper-input-label-1">Username</label>
            <iron-input id="input" slot="input" class="style-scope paper-input">
               <input id="nativeInput" class="style-scope paper-input" required="" autocomplete="on" placeholder="" autocapitalize="none" autocorrect="off" aria-describedby="paper-input-add-on-3" aria-labelledby="paper-input-label-1" pattern="[A-Za-z0-9._%+-]+@[A-za-z0-9.-]+\.[A-Za-z]{2,10}$">
            </iron-input>
         </div>
         <span class="suffix style-scope paper-input-container"></span>
      </div>
      <div class="underline style-scope paper-input-container">
         <div class="unfocused-line style-scope paper-input-container"></div>
         <div class="focused-line style-scope paper-input-container"></div>
      </div>
      <div class="add-on-content style-scope paper-input-container">
         <paper-input-error aria-live="assertive" slot="add-on" class="style-scope paper-input x-scope paper-input-error-1" id="paper-input-add-on-3">
            Please enter valid email id   
         </paper-input-error>
      </div>
   </paper-input-container>
</paper-input>    

我们正在使用Selenium Webdriver和Page Factory设计模式，我在BasePage类中有这个功能：

public IWebElement ExpandShadowRoot(IWebElement shadowRootElement)
{
   IWebElement shdowTreeParent = (IWebElement)((IJavaScriptExecutor)_driver)
            .ExecuteScript("return arguments[0].shadowRoot", shadowRootElement);
   return shdowTreeParent;
}

提前感谢您就此问题提供的帮助。