Polymer is not a polyfill for the standards. It's just a library for creating web components in a less boilerplatey manner. The polyfill library is called webcomponents.js.
If you wish to be precise, webcomponents.js was broken out of Polymer a while back. See e.g. [1]. In practice, if you use Polymer you are still using the polyfill + the elements from the catalog, as exemplified on [2].
you have to include the polyfills anyway, unless you are targeting something like Electron or an intranet where everybody is guaranteed to be using Chrome.
That's why I put "standard" in quotes; although it's been around for years, it's really only supported by one widely used browser.
[1] https://github.com/Polymer/HTMLImports
[2] https://www.polymer-project.org/1.0/