What?

Fixes a bug in dynamic imports that was not covered by e2e tests.

Why?

Modules from newly visited pages can have dynamic dependencies. As these pages can have different import maps, the modules are processed by the Interactivity Router to resolve their dependencies. The library on which our implementation is based uses a global function called importShim. That function is in our code, but it's not globally accessible by mistake.

This can cause client-side navigation to fail occasionally when the full-page client-side navigation experimental feature is enabled, showing this error in the console:

Uncaught (in promise) ReferenceError: importShim is not defined
    at @wordpress/interactivity-router (interactivity-router":1:1)
    at __webpack_require__ (bootstrap:19:1)
    at async wrapped (utils.ts:168:14)

Additionally, modules listed in the initial import map fail whenever a new module attempts to import them dynamically.

How?

Adding the missing function to self and initializing the modules that are present in the initial import map.

Testing Instructions

1. In the admin panel of your test site, go to Gutenberg > Experiments and enable the full-page client-side navigation feature.
2. Visit a page or a post on your site.
3. Click on the link that directs to the homepage (or any other page with a Query block containing pagination blocks).
4. Hover on the Next Page block.
5. Without the bug fix, the console error mentioned above appears. Ensure it doesn't appear, and the navigation was successful.

There are new e2e tests to cover the missing cases.