Revert "Analyze SDK with non-nullable experiment enabled, update MockSdk."

This reverts commit 545b80ef0e070841260132aac41bb2153fd079a0.

Reason for revert: this is a breaking change for MockSdk users, actually much more serious issue for un-forking SDK. https://github.com/dart-lang/sdk/issues/40500

Original change's description:
> Analyze SDK with non-nullable experiment enabled, update MockSdk.
> 
> Change-Id: I80264a6533045c33ed794a5938f6719f3b5a6d0b
> Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/134401
> Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
> Commit-Queue: Konstantin Shcheglov <scheglov@google.com>

TBR=paulberry@google.com,scheglov@google.com,brianwilkerson@google.com

Change-Id: If8c3d8322986b01012fa76f9c928f35309c00e3e
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/134802
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>