[native_assets_cli] HookOutput dependencies should not have to include any Dart files

[dcharkes]

The compiler should know what Dart source files have been used running a Dart script. So we shouldn't request hook writers to list transitively all Dart files in HookOutput.dependencies.

Currently the hook is run as dart path/to/some/hook/<hookname>.dart --config=path/to/config.json.

In order to get a list of all Dart files used, we have multiple options:

1. Manually compiling to kernel first, and having that output a .d file (not preferred, I'd rather not depend on SDK internals.).
2. Adding a --dependencies=path/to/deps.d to dart. dart --dependencies=path/to/deps.d path/to/some/hook/<hookname>.dart --config=path/to/config.json. This could be used for other use cases as well. (preferred)

Then we can also get rid of the weird dartBuildFiles for the CBuilder:

        final cbuilder = CBuilder.library(
          sources: [addCUri.toFilePath()],
          name: name,
          assetName: name,
          pic: pic,
          dartBuildFiles: ['hook/build.dart'],
        );

Thanks @kustermann!

[dcharkes]

@jensjoha can the CFE output a list of dart source files?

[jensjoha]

The frontend_server for instance lists the keys in component.uriToSource (as in +file1\n+file2 etc). Is that what you want? (I seem to be missing some context here).

[mkustermann]

We could even make flutter build and dart build:

• Figure out transitive package dependencies
• Find all the N hook/* scripts.
• Compile all of them to kernel: We could do that using P parallel incremental compilers that are given the N scripts to compile
• A compilation will emit kernel and a .d file containing dependencies
• We do the compilations in DFS order
• As soon as a kernel compilation has finished we can run it (assuming dependencies are ready)

Most of our tools already support emitting .d dependency files (e.g. dart --snapshot-kind=kernel --snapshot=test.dill --depfile=test.dill.d test.dart, frontend-server, dart2js, dart2wasm, ...) - because flutter already needs those dependency files to detect whether to rebuild things.

We should be really optimizing for compile times in dart build / flutter build, e.g. if the package graphs depth is 30 and all of them use hooks, saving a single second (e.g. via parallel compile of hooks to kernel) would save half a minute for a developer.