Support the --{no-}sound-null-safety flags in the kernel worker

[jakemac53]

The kernel worker (used by build_runner and google3) does not currently plumb through or support the --{no-}sound-null-safety flag, this will be a requirement for running in sound null safety mode with our modular builds.

There are some open questions as to exactly how it should be implemented:

• Should we throw away all cached modules if the flag changes from one invocation to the next?
  • If we do this then probably blaze and build_runner would run twice as many workers, keeping a set for each mode and delegating requests to the respective workers.
• Or should we keep two incremental compilers around, and two caches of components, so that we can more efficiently switch back and forth?

I believe DDC already supports this flag, but I don't know how it is handling invalidation issue listed above, cc @nshahan @Markzipan

cc @jensjoha @sigmundch @natebosch @davidmorgan

[jakemac53]

assigned to the area-front-end for right now for a lack of a more obvious area to assign it to

[nshahan]

I believe DDC already supports this flag, but I don't know how it is handling invalidation issue listed above

I haven't tested it properly in the worker configuration but I added checks to invalidate if the mode changes and to error if you try to compile with --sound-null-safety and use weak summary .dills for the sdk or dependencies.
https://dart-review.googlesource.com/c/sdk/+/146962/17/pkg/front_end/lib/src/api_unstable/modular_incremental_compilation.dart

I might have missed something though if we find the invalidation isn't enough.

[jakemac53]

Nice @nshahan that is a fine option I think - and that is shared code so we just need to plumb the flag through then. I can work on that.

[jakemac53]

This was resolved in 1522a86