Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed (or fixed any issues), please reply here (e.g. I signed it!) and we'll verify it.

What to do if you already signed the CLA

Individual signers

• It's possible we don't have your GitHub username or you're using a different email address on your commit. Check your existing CLA data and verify that your email is set on your git commits.

Corporate signers

• Your company has a Point of Contact who decides which employees are authorized to participate. Ask your POC to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the Google project maintainer to go/cla#troubleshoot (Public version).
• The email used to register you as an authorized contributor must be the email used for the Git commit. Check your existing CLA data and verify that your email is set on your git commits.
• The email used to register you as an authorized contributor must also be attached to your GitHub account.

ℹ️ Googlers: Go here for more info.

CLAs look good, thanks!

ℹ️ Googlers: Go here for more info.

Just to note, the docs currently recommend returning null in one specific (probably very rare) circumstance--if you're extending an element with a template, but you don't want your element to have one. https://polymer-library.polymer-project.org/3.0/docs/devguide/dom-template#elements-with-no-shadow-dom

If the strict template changes changed this behavior, we should update the docs, but pinging @kevinpschaaf here in case he has input.

As I read it, this method can return either undefined or null.

undefined if the mixed-in element doesn't have a template, either as a dom-module or inline.

null if the mixed-in element explicitly declares a null template, as we recommend that you do if it's an element with no template (like e.g. iron-ajax)

I'd prefer:

{string|!HTMLTemplateElement|null|undefined}

Done. PTAL

HTMLTemplateElement without ! implicitly means nullable, so I think just removing the ! from the setter parameter type comment is the way to go. Changed both to {HTMLTemplateElement|string}. PTAL