Create a unified SourceMap class

[DartBot]

<img src="https://github.com/avatars/u/188?v=3" align="left" width="96" height="96"hspace="10"> Issue by nex3
Originally opened as dart-lang/sdk#13805

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Right now, when building a source map, you can only get a map or a JSON string. When parsing the map, though, you get an instance of the Mapping interface, which provides a few APIs for inspecting the contents but no way to re-serialize the map.

It would be preferable to have a single class representing a source map that can be serialized and inspected. It would also be good to have a more thorough inspection API than Mapping currently provides -- at the very least a way to get a list of all the segments in the source map, a way to access/modify the source files, etc.

[DartBot]

<img src="https://github.com/avatars/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.

[DartBot]

<img src="https://github.com/avatars/u/613610?v=3" align="left" width="48" height="48"hspace="10"> Comment by kegluneq

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Submitted dart-lang/sdk@890433d as a start to this.