Benchmarks: Record Dart VM time with JIT'd/warm code

[matanlurey]

At least internally, we spin up a worker and use it for incremental rebuilds. This means the code/snapshot is already loaded, already JITd, and kept warm for the development cycle. This is more likely than spinning up to do a one-time build and closing.

[nex3]

I'm skeptical that it's more likely—I think most users of the Dart VM version will run it from the CLI rather than setting up a Dart code harness for it. Maybe once we have some watcher functionality this will be more relevant?

I'm not opposed to adding this to the performance document, but I don't have time for it personally.

[matanlurey]

I don't mind helping here! Internally and with Bazel we'd have it as a worker, so I can contribute.

Edit:

@nex3 Would you prefer a separate repository for Bazel rules and a worker implementation? i.e. dart-sass-bazel, or just part of this repository? I can link you internally to what it would look like.