It seems like you should be able to say parse(json).urls to get all the source URL's, regardless of whether it's a SingleMapping or a MultiSectionMapping. To support this, Mapping should have an abstract "urls" getter which is implemented by MultiSectionMapping to combine all the url lists from the individual mappings.
Workaround: implement this yourself; it's not that hard.
DartBot
commented
9 years ago 
Bug reported by @skybrian in the old github repo (https://github.com/dart-lang/source-maps/issues/3)
Original bug description:
It seems like you should be able to say parse(json).urls to get all the source URL's, regardless of whether it's a SingleMapping or a MultiSectionMapping. To support this, Mapping should have an abstract "urls" getter which is implemented by MultiSectionMapping to combine all the url lists from the individual mappings.
Workaround: implement this yourself; it's not that hard.