Source-maps can be linked to from a source-file by a comment at
the end of the file.
Previously the `SourceFileLoader` would read
the first comment that matched `//# sourceMappingURL=` but
this is not valid since some bundlers may include embedded
source-files that contain such a comment.
Now we only look for this comment in the last non-empty line
in the file.
PR Close#32912