Framework: Add wp-polyfill as central polyfill

[aduth]

Related (will require changes to): #9750

This pull request seeks to refactor our polyfills, introducing a new central wp-polyfill handle which includes polyfills for both JavaScript language features (Promise, etc) and modern browser features (currently window.fetch, Node#contains, FormData, soon Element#closest).

Implementation notes:

It does not remove or deprecate wp-polyfill-ecmascript, but rather acts as a superset, where the intended contract of "ecmascript" would be for language features, regardless of whether the underlying babel-polyfill / core-js provides browser polyfills.

Testing instructions:

Verify there are no regressions in the behavior of polyfills, particularly in Internet Explorer 11.

Note that there is currently a separate issue with saving in IE11, not addressed here.

[youknowriad]

In one of my plugins, I was using this technique but I recall it was creating an issue for the script loading order. I can't reproduce thought here but thought I'd mention.

[aduth]

Hmm, okay, it's good to note. I've not encountered an issue like that which you describe. I did find that wp_add_inline_script doesn't work with the null $src, which is why I've opted for the equivalent wp_script_add_data below.

[youknowriad]

Weren't you planning to add element closest or are you leaving this for a separate PR?

[aduth]

Weren't you planning to add element closest or are you leaving this for a separate PR?

If we can review and merge #9750 before this one, I can rebase and add it.

[youknowriad]

Seems like the opposite is simpler right?

[youknowriad]

LGTM 👍

[afercia]

Testing on IE 11, here's what I see:

first one wp-polyfill-fetch.6bd312cf.js (78,1) seems it's about the line 78
export function Headers(headers) { ...

I guess IE 11 doesn't know what export is. Really not my area of expertise, but shouldn't these files be transpiled before being served to IE 11?

Not sure about the error related to fetch. Expanding the error details in the IE 11 console doesn't give any useful hint, but I guess it's related to the above error.

[aduth]

@afercia The issue you're seeing there is addressed by #9795, which has since been merged. If you git fetch origin and git rebase origin/master your local copy of this branch, the issue should go away. I'm verifying this myself at the moment too.

[afercia]

@aduth thanks will check. Note: I've also seen Object doesn't support property or method 'includes' when activating Yoast SEO and the Classic Editor plugin, it may be unrelated though. Will investigate.

[aduth]

In my testing after rebasing with the included #9795 fix, everything here works as expected in IE11.

[afercia]

Yup I don't see the error related to fetch any longer.