Fix emitting real mapping sources when null mapping comes first

[jridgewell]

This fixes an ordering bug when a null mapping and a real mapping share the same sourceIndex. If a null mapping comes first, then we'll assign it a sourceIndexToSourcesIndex but skip recording its source files in the items array because of the isNullEntry check. When we find the real mapping later in results, we'll skip recording its source files because of the map-ok check.

The simplest fix I can think of is flipping the order just a bit. We'll skip assigning sourceIndexToSourcesIndex for null mappings, so that when a real mapping comes up we'll record its sources.

I'm not sure how to generate a unit test case for this, but it resolves the issue found in angular/angular-cli#29465 (comment).

Fixes #4080
Fixes #4107

[meriturva]

No chance that this PR will be merged?
Thank you!

[evanw]

Thanks for the PR! I'll take care of reducing the repro and adding a test case. The linked Angular issue is very helpful as I'm able to reproduce the problem myself. A minimal repro is pretty complicated and may involve at least four files.

It looks like the issue happens when the same source file emits multiple discontiguous parts in the output file and source maps are skipped for the first part. This can happen when esbuild automatically generates a separate __export(exports, { ... }) part for the file to handle ESM namespace exports, for example.

Looks like this also fixes #4107.

Edit: For my own reference, here is a minimal repro for this bug. Clicking Visualize this source map on the generated file shows that the source map is invalid.