Framework: Use standard polyfill pattern for Element#closest polyfill

[aduth]

Related: #7033 (specifically overview at #7033 (review) )

We should consolidate our approaches to polyfilling missing behavior, mostly impacting Internet Explorer support. Our use of the element-closest module has proven difficult to maintain. The approach used in #7033 for polyfilling Node#contains is looking to be a better candidate for its maintenance and performance implications.

As part of this, we must determine a way to ensure these polyfills are loaded by all dependent scripts. Some ideas:

In that sense, and given that these are true progressive enhancements which do nothing if a browser is detected to support a given feature, I'd say: Load it everywhere; every WordPress admin screen; for any features we ever use.

#7033 (review)

we should probably not need to load it if:

• There are zero scripts on a screen otherwise
• We have some way of knowing / inferring that modern authoring practices are employed
  • Only those which are transpiled (from a build directory)?
  • Some other metric akin to @babel/preset-env@next's useBuiltIns: 'usage'?
  • Register scripts through a new abstracted registration function, e.g. wp_register_script_with_polyfill?

#7033 (comment)

There's a couple things I'd like to see, but not necessarily as part of these changes:

• Avoid polyfills being considered as dependencies of specific scripts, unless we're certain we can maintain those dependencies.
  • There's nothing about wp-dom that depends on the Node#contains polyfill. Wherever we're using it should be the dependency
  • But then again, this highlights the bigger problem that this is a source of maintenance headaches and is likely not worth maintaining, vs. load always
• Better names for the script enqueues, which avoids potential plugin incompatibilities. Probably something with a WP-specific prefix, e.g. wp-polyfill-node-contains

#7033 (review)

[aduth]

Related Slack conversation (Core JS Meeting, 2018-06-19):

https://wordpress.slack.com/archives/C5UNMSU4R/p1529414641000073

(Link requires registration)

[cristianozanca]

Hi,
just found this error, using Gutenberg - Yoast - Woo Gutengerg Block plugins
Uncaught Error: only one instance of babel-polyfill is allowed http://wordpress.woo/wp-content/plugins/wordpress-seo/js/dist/commons-820.min.js?ver=8.2 line 1

[aduth]

Hi @altrovideo , thanks for the report. You can follow Yoast/wordpress-seo#10969 for Yoast's work on this. It appears to have been merged, so should be resolved in an upcoming release.