Support custom asset types 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#14966

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Users have expressed an interest in defining custom asset types. This would require some sort of pluggable serialization system in order to send these assets between isolates. Alternately, if we say these assets can only be used by transformers in the same library where they're defined, we could potentially find a way around serializing them.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/2049220?v=3" align="left" width="48" height="48"hspace="10"> Comment by sigmundch

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+1 on finding a way to avoid serializing them if possible.

Pete recently also expressed interested in this. Here is an example use case: we want to be able to parse a dart file and store an AST as a custom asset so we don't have to repeat the parsing later on.

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cc @blois.

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<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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Would the AST be passed between different transformers, or cached for re-use in a single transformer?

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<img src="https://avatars.githubusercontent.com/u/2049220?v=3" align="left" width="48" height="48"hspace="10"> Comment by sigmundch

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Different transformers. Ideally these could be completely separate transformers. But I have also scenarios where they could be part of the same transformer group. So those would at least be being part of the same isolate.

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<img src="https://avatars.githubusercontent.com/u/2152569?v=3" align="left" width="48" height="48"hspace="10"> Comment by blois

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Scenario that I was looking at was a tweak system with support for compiling out the tweaks in release mode. With a series of code transformations such as this, would be nice to not have to re-parse the AST.

Potentially the answer is that there should be a single code-transformer which executes a series of AST modifiers. But I imagine that this same scenario could be generalized.

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<img src="https://avatars.githubusercontent.com/u/4865287?v=3" align="left" width="48" height="48"hspace="10"> Comment by lrhn

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Removed Area-Library label.
Added Area-pKG label.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/4865287?v=3" align="left" width="48" height="48"hspace="10"> Comment by lrhn

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Removed Area-pKG label.
Added Area-Pkg 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.