[ddc] Migrate dev_compiler library to null safety
[ddc] Migrate dev_compiler library to null safety

Issue: https://github.com/dart-lang/sdk/issues/46617
Change-Id: I87d8bcb65e22f4d25cc24694ababa1a16d42bdd7
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/250348
Reviewed-by: Mark Zhou <654f5fe0c4ca6c799b0e735dcf3fc1c389f35bf9@google.com>
Commit-Queue: Nicholas Shahan <03847abd2d8aadfe859ecb98095a31f9f515dc47@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk