[cfe] Set and check null safety mode in .dill

Validates that the .dill files provided for dependencies and the SDK
support sound null safety when compiling for strong mode.

Change-Id: Id1f7547a6bc473b901bfce62145cc918353c8ea4
Issue: https://github.com/dart-lang/sdk/issues/41694
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/149080
Reviewed-by: Jens Johansen <jensj@google.com>
Commit-Queue: Nicholas Shahan <nshahan@google.com>

https://dart.googlesource.com/sdk/+/2cc4d7ca2f932e732e81ee931c215f4f97eb8f19
1 file changed
tree: fe52689f45a4c8d913b8cd19397f8c70bc4de22d
  1. commits.json