Previously when rendering flattened source-maps, it was assumed that no
mapping would come from a line that is outside the lines of the actual
source content. It turns out this is not a valid assumption.
Now the code that renders flattened source-maps will handle such
mappings, with the additional benefit that the rendered source-map
will only contain mapping lines up to the last mapping, rather than a
mapping line for every content line.
Fixes#35709
PR Close#35718
bfe7657006