Update check_nnbd_sdk to diff expectations directly in dart.
Update check_nnbd_sdk to diff expectations directly in dart.

This makes the script properly produce an error message when
it detects a mismatch on windows (there `diff` is not available.)

Change-Id: Ibefe8c69023b42e8710e744f88257ff967d34bd2
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/134094
Reviewed-by: Vijay Menon <671ecbb18d31a1c038875027dcfe976af959ea9f@google.com>
Reviewed-by: Nicholas Shahan <03847abd2d8aadfe859ecb98095a31f9f515dc47@google.com>
Commit-Queue: Sigmund Cherem <beaea8fd7d2a56555046ba4f362c8a93e3938576@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