consider providing unique identifiers that transformers can use for generating files

[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#12618

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It's important for transformers that generate new files to choose a file name that doesn't collide with existing files or files generated by another transformer that runs on the same phase.

We are not sure how much this will be a problem in the future, but one way to reduce the risk of name collisions would be to provide APIs in barback for this. For example, an API to retrieve a unique string or unique ID corresponding to the current transformer at the current phase. Then the transformers can generate unique assets IDs for generated files by including this unique string somewhere in the path.

(more context available in the discussions of this CL https://chromiumcodereview.appspot.com/22825012/)

[DartBot]

<img src="https://avatars.githubusercontent.com/u/3276024?v=3" align="left" width="48" height="48"hspace="10"> Comment by anders-sandholm

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Removed Library-Barback label.
Added Pkg-Barback label.