Fix race condition where last format transform overrides previous ones

[m-mujica]

The transform function resolved by transformInput was handling the bundle concatenation to the event loop too early, causing future transforms (sharing the same graph) to reset the activeSourceKeys object -where the result of transpiled graphs are kept- before the bundle was concatenated.

This meant the last transform would override all of the previous ones, instead of doing return Promise.resolve().then(concatenateBundle), this commit creates a new promise that runs concatenateBundle right away instead of waiting for the next tick (and opening the window from future transforms to mutate the graph).

Closes #1139

[m-mujica]

[matthewp]

lgtm, awesome work