The library used by ngcc to update the source files (MagicString) is able
to generate a source-map but it is not able to account for any previous
source-map that the input text is already associated with.
There have been various attempts to fix this but none have been very
successful, since it is not a trivial problem to solve.
This commit contains a novel approach that is able to load up a tree of
source-files connected by source-maps and flatten them down into a single
source-map that maps directly from the final generated file to the original
sources referenced by the intermediate source-maps.
PR Close#35132
Pete Bacon Darwin