* Throw a specific error on mixed compilation mode; let the incremental
compiler ignore that error when trying to initialize (i.e. it doesn't
initialize from it, but it doesn't show any warning either).
* Allow some mixed mode stuff: Eventually the SDK should be in agnostic
mode, so we should allow mixing agnostic with non-agnostic.
Fixes#41493.
Change-Id: Idb33fb31afe6bbba6d74134cb722ca825751898b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/143583
Commit-Queue: Jens Johansen <jensj@google.com>
Reviewed-by: Johnni Winther <johnniwinther@google.com>