[assets] The asset builder system should compile hook/{build,link}.dart once.

[mkustermann]

It seems a flutter build apk --release will compile the hook/build.dart script 3 times. Though a hook/build.dart compiled to build.dill doesn't depend on the build config and therefore shouldn't be in the build-config specific folder.

These are two separate concepts:

• Caching of kernels of dart scripts to make hook invocations faster
• Running build/link hooks with different build configurations

One could even argue that the native asset builder system should use dart run to invoke hooks (as the hook source code may transitively use assets on it's own) and the dart run tool should be responsible for optimizing repeated runs of the same script with same Dart SDK version.

But if we do it ourselves - we should avoid compiling it several times and putting it in a location that cannot be shared across runs.