Let barback tell a transformer which inputs were modified

[DartBot]

<img src="https://avatars.githubusercontent.com/u/46275?v=3" align="left" width="96" height="96"hspace="10"> Issue by munificent
Originally opened as dart-lang/sdk#18301

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One of the Angular transformers does whole program analysis using the Analyzer. This is pretty time-consuming. To minimize that, it preserves the resolved state across calls to apply().

When apply() is called, it needs to update that state to take into account the inputs that were modified. To minimize the amount of re-analysis it does it wants to only tell the analyzer about the inputs that were changed. Currently, barback doesn't tell the transformer, so it resorts to reading all of the inputs and comparing their contents as strings (!).

Since barback does know that, it would be good to expose that in the Transform API.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/46275?v=3" align="left" width="48" height="48"hspace="10"> Comment by munificent

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Added Duplicate label.
Marked as being merged into dart-archive/barback#35.