Support lazy compilation in barback

[nex3]

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.

[nex3]

Marked this as blocking #16076.

[nex3]

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 #16076 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.

[anders-sandholm]

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

[nex3]

Fixed by r32516.

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Added Fixed label.

[DartBot]

This issue has been moved to dart-archive/barback#26.