Allow transformers to generate/modify code in packages as a result of whole-code evaluation

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<img src="https://avatars.githubusercontent.com/u/2152569?v=3" align="left" width="96" height="96"hspace="10"> Issue by blois
Originally opened as dart-lang/sdk#17306

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Right now transformers can only modify code in packages when that package is being transformed, but there are a number of scenarios where the transformation on the package needs to be done with the results of some whole-program analysis.

The specific scenario I'm running into is generating code for Angular in a lazy-loading environment. In a non-lazy environment the generated caches are all injected into the application entry point, but in the lazy-loaded scenario the code needs to be injected into the root of the lazy library.

The issue is that the calculation of what needs to be cached needs to take the entire app into account.

One approach discussed is the 'global transformers' which execute after all package transformers and can modify every package contents.

Marking as critical as it is required for Angular's transformers once lazy-loading is supported.

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<img src="https://avatars.githubusercontent.com/u/46275?v=3" align="left" width="48" height="48"hspace="10"> Comment by munificent

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Issue dart-lang/pub#670 has been merged into this issue.

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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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Removed Area-Pub, Pub-Build labels.
Added Area-Pkg, Pkg-Barback labels.

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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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Removed Priority-Critical label.
Added Priority-Medium label.

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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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FYI - the latest update is that we'll try to do this as a special transformer that creates a copy-on-write of assets. More details here: https://code.google.com/p/dart/issues/detail?id=18489

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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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We're going to experimentally try an API that allows an application to declare transformers for its dependencies. The syntax will be something like this:

    experimental.global_transformers:
    - some_transformer
    - another_transformer:
        config: value

Each transformer is loaded and applied to each package's asset cascade in order after the normal transformer loading has finished.

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<img src="https://avatars.githubusercontent.com/u/522948?v=3" align="left" width="48" height="48"hspace="10"> Comment by justinfagnani

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Interesting!

Not to look a gift horse in the mouth, but I think that we often need the other direction - that it when a library declares a transformer it needs to transform the entrypoints as well. This is true for libraries that need to register or initialize like custom elements, js-interop, polymer expressions, etc.

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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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I think the way to handle those might be requiring the entrypoint to add a transformer to its global transformer list.

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<img src="https://avatars.githubusercontent.com/u/522948?v=3" align="left" width="48" height="48"hspace="10"> Comment by justinfagnani

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Which is what we currently do. What's unfortunate about that is that it's hard to document. Some transitive dependency uses Polymer and then the app needs to declare the transformer.

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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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If a transitive dependency needs to change code in an application, I think it's reasonable for that application to explicitly opt into that dependency's modifications. Letting them happen silently is a recipe for confusing non-local effects, even if we could deal with the ordering concerns.

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<img src="https://avatars.githubusercontent.com/u/405837?v=3" align="left" width="48" height="48"hspace="10"> Comment by zoechi

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My package needs a transformer (DI) for examples only and causes troubles (a bug in DI though) in an app that has my package as a dependency because the transformer is executed during pre-compilation and no way to opt out.

#­10 see http://dartbug.com/21777

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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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If you only include the example directory, it shouldn't ever get instantiated for a dependency:

    transformers:
    - di:
      $include: example/**

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<img src="https://avatars.githubusercontent.com/u/405837?v=3" align="left" width="48" height="48"hspace="10"> Comment by zoechi

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Thanks, that's actually a good idea. Didn't think of it.