[record_use] Dart API: Loading units as object/class #2979

We should consider making loading units an object in the Dart API, that way in the future we could potentially add extra fields such as a fully qualified file name.

Reasoning: Some use case you use the record-use not from the context of a link hook, but because you wrapped the Dart compiler. E.g. dart compile js --resources-file. If the Dart compiler outputted multiple loading units, and you want to put the assets with the right loading units, simply knowing the name of the loading unit might be unpractical.

(We might not need extra fields in the loading unit, but it would be unfortunate if we paint ourselves into a corner.)

Corresponding JSON encoding issue:

• [record_use] Implementation: JSON: Loading units as object #2980