polymer.concat.js:9118 Uncaught DOMException: Failed to execute 'createShadowRoot' on 'Element': Shadow root cannot be created on a host which already hosts a shadow tree.
    at HTMLElement.shadowFromTemplate (http://www.core-async.info/components/polymer/polymer.js:14:418)
    at HTMLElement.parseDeclaration (http://www.core-async.info/components/polymer/polymer.js:14:257)
    at HTMLElement.parseDeclarations (http://www.core-async.info/components/polymer/polymer.js:14:160)
    at HTMLElement.makeElementReady (http://www.core-async.info/components/polymer/polymer.js:13:31411)
    at HTMLElement.createdCallback (http://www.core-async.info/components/polymer/polymer.js:13:30990)
    at HTMLElement.registerPrototype (http://www.core-async.info/components/polymer/polymer.js:14:13189)
    at HTMLElement.register (http://www.core-async.info/components/polymer/polymer.js:14:11151)
    at HTMLElement._register (http://www.core-async.info/components/polymer/polymer.js:14:16096)
    at Object.flush (http://www.core-async.info/components/polymer/polymer.js:14:14764)
    at Object.ready (http://www.core-async.info/components/polymer/polymer.js:14:14879)