Barback should ensure that assets are properly finalized

[DartBot]

<img src="https://avatars.githubusercontent.com/u/188?v=3" align="left" width="96" height="96"hspace="10"> Issue by nex3
Originally opened as dart-lang/sdk#14188

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When Barback passes Asset objects to Transformers via [isPrimary] and [apply], it's possible for those Transformers to open the Asset's stream for reading without properly closing it afterwards. This can create resource leaks that will become a problem in a long-running "pub serve" process. Since barback knows when an Asset is no longer in use by a transformer, it should close those Assets' streams.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/188?v=3" align="left" width="48" height="48"hspace="10"> Comment by nex3

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Marked this as blocking dart-lang/pub#647.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/188?v=3" align="left" width="48" height="48"hspace="10"> Comment by nex3

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After investigating this further, I don't think it's likely for a transformer to cause an asset's content not to be garbage collected. I've tried several ways of reproducing this locally, but none of them seem to have memory consumption patterns that relate to whether or not transformers leave streams unread or partially-read. Barback clearly has memory leak issues, but I don't think this is the culprit.

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Added Invalid label.

[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.