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?