refactoring source_maps

[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#19930

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source_maps was a kitchen sink before we had other uses for what we have there. It should probably be refactored as follows:

 - span - should be its own pacakge
 - source_maps parser & builder should stay
 - refactor & printer - should go to code_transformers?

[DartBot]

<img src="https://avatars.githubusercontent.com/u/2049220?v=3" align="left" width="48" height="48"hspace="10"> Comment by sigmundch

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+Nathan, since he is already looking into factoring Span into its own package.

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cc @nex3.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/188?v=3" align="left" width="48" height="48"hspace="10"> Comment by nex3

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Set owner to @nex3.
Added Started label.