
this.importHref is not available anymore in Polymer 2. You have to use Polymer.importHref instead. After consulting Polymer experts the best way to create a hybrid code base is to use a simple switch: (this.importHref || Polymer.importHref) See also the Polymer 2 upgrade docs: https://polymer-library.polymer-project.org/2.0/docs/upgrade#importhref Change-Id: I0ab3e9ad95528989208f650fdd9876d1399b031d