add Mapping.urls property to sourcemaps

[DartBot]

<img src="https://avatars.githubusercontent.com/u/2049220?v=3" align="left" width="96" height="96"hspace="10"> Issue by sigmundch
Originally opened as dart-lang/sdk#13194

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Bug reported by @­skybrian in the old github repo (https://github.com/dart-lang/source-maps/issues/3)

Original bug description:

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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.

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[DartBot]

<img src="https://avatars.githubusercontent.com/u/3276024?v=3" align="left" width="48" height="48"hspace="10"> Comment by anders-sandholm

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Removed Library-SourceMaps label.
Added Pkg-SourceMaps label.