The HTML Imports polyfill has moved to https://github.com/webcomponents/webcomponentsjs
For more information, see: http://webcomponents.org/polyfills/
HTML Imports are a way to include and reuse HTML documents in other HTML documents. As <script> tags let authors include external code in their pages, imports let authors load full HTML resources. In particular, imports let authors include Custom Element definitions from external URLs.