Add link.dart hook

[mosuem]

Support a hook/link.dart script, which executes after kernel compilation. The SDK changes to make this work are in https://dart-review.googlesource.com/c/sdk/+/338380.

The link script receives as input a list of assets produced by hook/build.dart scripts. In a hook/build.dart, users can specify if an asset should be sent to a link script, and give the name of the package containing that script. All assets destined for a particular link script are collected and given as input.

In #1117, the link scripts get as additional input tree shaking information collected during kernel compilation, to enable link scripts to remove or add assets based on usage of symbols in Dart code.

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[dcharkes]

I think the most important high level questiosn we have to answer before settling on implementation details is:

• What information do build.dart and link.dart need that they don't share? And what output don't they share? -> We shouldn't provide classes with fields that are unused in a certain scenario. Neither json/yaml fields that are ignored in one of the steps. That's bad for documentation, and also exposes too many internal details.
• How does the use case work where build.dart of one package outputs something that is used by another's package link.dart? How do we ensure the untreated asset from build.dart is not added to the final Dart application? -> We should have an example here in the repo that uses that use case.
  • As a sub question: Do we ever want to pass non-assets from build.dart to link.dart (see meta-data between build.dart invocations)? Or should users just serialize such data into a json/yaml and pass it as a data asset?
• How do we distinguish between different asset types (native code, data, wasm code) in the BuildOutput and LinkOutput. For example flutter_tools for Android needs to tell gradle what jni-dylibs to link, but presumably needs to copy data files into a separate location.

[dcharkes]

LGTM with https://dart-review.googlesource.com/c/sdk/+/338380/39/pkg/dartdev/lib/src/commands/build.dart addressed!

[dcharkes]

TODO: Rename build_{runner/planner}.dart to {runner, planner}.dart

Can you update the PR description with a list of changes?

[dcharkes]

Ship it! 🚢

Please wait with merging until https://dart-review.googlesource.com/c/sdk/+/365780 is in, in case we need to land other fixes to make that roll go through.