title: How can I get polymer
body: <p>I have tow repos(A and B, B depends on A).Now A has been upgrade to Polymer@3 and B still use Polymer@2.
  I use <code>import { PolymerElement } from '@polymer/polymer/polymer-element.js';</code> in A and <code>class AgeSwitch extends Polymer.Element</code> in B.
  Before upgrating to Polymer@3,B can get <code>Polymer</code> by A.But now it is undefinded.How can I get <code>Polymer</code> or <code>PolymerElement</code>?Thanks.</p>