[ci skip] prepare v1.12.0

bower.json

@@ -1,6 +1,6 @@
 {
   "name": "polymer",
-  "version": "1.11.3",
+  "version": "1.12.0",
   "main": [
     "polymer.html",
     "polymer-mini.html",

package.json

@@ -1,6 +1,6 @@
 {
   "name": "@polymer/polymer",
-  "version": "1.11.3",
+  "version": "1.12.0",
   "description": "The Polymer library makes it easy to create your own web components. Give your element some markup and properties, and then use it on a site. Polymer provides features like dynamic templates and data binding to reduce the amount of boilerplate you need to write",
   "main": "polymer.html",
   "directories": {