Summary:
This diff is part of a stack that re-structures the metro-transform-worker with the goal to make the transformation pipeline(s) more visible as well as what metadata is added by each step. The ultimate goal is to model the pipeline as a set of transformation-steps where the output of one step is the input of the next step (e.g. the asset step transforms an asset to a JS artifact that than is transformed by the JS step).

This diff extracts the json transformation into its own function. It does not yet call into the JS-transformation because that potentially changes the behavior (this diff is a "mechanical refactor").

Reviewed By: motiz88

Differential Revision: D28386025

fbshipit-source-id: 371421cfae99ea6e6cc81efe9b55c86fa44e92ce