Previously: #19871
Related: https://github.com/Financial-Times/polyfill-library/issues/462, #20172 (comment), Trac#49360

This pull request seeks to resolve issues where the current polyfill for URL introduced in #19871 is ineffective:

• The condition will never be satisfied, since window.URL does exist in IE11. It's specifically the constructor which is not supported (see related comment).
• The chosen polyfill is non-spec-conformant in a way that we need it to be (https://github.com/Financial-Times/polyfill-library/issues/462, URL: Conform to URL Living Standard definition of valid URL #19871)

The proposed changes improve the condition, and use core-js-url-polyfill in place of polyfill-library for the URL polyfill. This is a package I've published to NPM, and is a simple wrapper of core-js's URL polyfills. See Trac#49360 comment for more context on why a wrapper module is needed.

Testing Instructions:

NOTE: The following can't yet be tested, because there is an unrelated error in IE11 preventing the page from being loaded. (TBD PR)

Prerequisites:

• Use Internet Explorer
• Either run WordPress 5.3.x (or any version earlier than 5.4-beta / trunk), or comment this line in the local copy of your branch:

  • gutenberg/lib/compat.php

    Line 55 in 5a5564b

    return;

  • This is needed because Gutenberg's polyfill script is not registered if there is already one registered for wp-polyfill-url. Since this was already patched to core in 5.4-beta, the changes would not otherwise take effect.

Test:

1. Navigate to Posts > Add New
2. Open F12 Developer Tools Console
3. Observe success: new URL( 'http://example.com' );
4. Observe thrown error: new URL( 'invalid' );