HTML Imports are a way to include and reuse HTML documents in other HTML documents. As <script> tags let authors include external Javascript in their pages, imports let authors load full HTML resources. In particular, imports let authors include Custom Element definitions from external URLs.
Include the html_import.debug.js or html_import.min.js (minified) file in your project.
<script src="packages/html_import/html_import.debug.js"></script>
html_import.debug.js is the debug loader and uses document.write to load additional modules.
Use the minified version (html_import.min.js) if you need to load the file dynamically.
For HTML imports use the import relation on a standard <link> tag, for example:
<link rel="import" href="import-file.html">
You can read more about how the polyfill is implemented in JavaScript here: https://github.com/Polymer/HTMLImports/tree/master#polyfill-details
This package is built from: https://github.com/Polymer/HTMLImports/tree/master
You'll need node.js to rebuild the JS file. Use npm install to get dependencies and grunt to build.