Support lazy compilation in barback

[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#16077

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Barback should have the ability for a transformer to declare that it should be run lazily -- that is, only if its assets are actually requested, either by another transformer or by the user via [getAssetById] or [getAllAssets].

In order to do this, the transformer will need to be able to run enough to say which assets in can produce, without actually producing the assets.

[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#815.

[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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Ideally, this would include the ability for a lazy transformer's outputs to be consumed by another lazy (or lazy-compatible) transformer without ever materializing them. They could also be rejected by a transformer's isPrimary even if it isn't explicitly lazy-compatible -- it would just have to not try to access the asset's contents.

In order to just get issue dart-lang/pub#815 solved, though, it should be sufficient to just have lazy assets be materialized if any other transformer has anything to do with them. This means that a lazy transformer will effectively only work if it's in the last phase of a cascade.

[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.

[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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Fixed by dart-lang/sdk@01cadd4.

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Added Fixed label.