Measure impact of transpiling

[romaricpascal]

Part of #3279

Note This will only apply if/once we introduce Babel to transpile our modern code to a syntax supported by all the browsers we decide to support

What

Measure the impact of transpiling modern syntax and display that measure somehow (review app + PR comment).

Why

The stats we'd gather with rollup-plugin-visualizer would highlight the share polyfills have in our built files. However, it doesn't say how much transpiling modern syntaxes to older one cost our users. Some of the transformations can add a good deal of extra code (and polyfills), so it's worth keeping an eye on that.

Who needs to work on this

Developers

Who needs to review this

Developers

Done when

• We've investigated how we may measure the impact of transpiling ¹
• We've implemented that measurement
• The measurement is accessible either on the review app, as a PR comment (or both)

Footnotes

1. Comparing the source file with the built file may seem like an easy option, but that'd miss all the files that get imported by the source file. We may be able to get some insight using the values reported by rollup-plugin-visualizer (eg. compare the sum of non-polyfilled sizes there with the size of the file on disc). ↩