[building][rollup] Modules imported to main module not appended at `rel=preload`

When building for modern and legacy and relying on dynamic imports, it is possible to for Rollup to create a chunk that is required in both the main module and the dynamically imported module, in this case the chunk is not registered as isEntry and will not be added to the HTML via <link rel="preload" />.

Reproduction steps:

1. Clone https://github.com/Westbrook/minesweeper/tree/2bec691f54708e3bb6eb83755139ffbd144c9a20
2. Install with yarn
3. Build yarn build
4. See lack of secondary <link rel="preload" />

Expected results: The <link rel="preload" /> is available and ensures the earliest request for this asset.

Actual results: There is no <link rel="preload" /> and the developer needs to add it manually for optimal performance.

[LarsDenBakker]

There is also https://developers.google.com/web/updates/2017/12/modulepreload which apparently on some browser traverses dependencies. I haven’t tried it yet

[LarsDenBakker]

Ah I see, so it was already broken 😃 Definitely an improvement then.

I think the different is that preload only preloads the network call, whereas modulepreload also parses the code (and potentially start downloading transitive dependencies?). In this case that’s not really needed

For now we can stick to preload as far as I can tell…