Revert "Tweak `expect.dart` library."

This reverts commit ff5f391c0a0247f449a959797ecf6e10ba950da2.

Reason for revert: The expect library is used by Flutter engine, and some of its tests use assertStatementsEnabled. There should be a migration path that doesn't require an atomic change, like adding the replacement api before removing the old one.

Original change's description:
> Tweak `expect.dart` library.
>
> Make API more consistent for a few methods.
> Reduce the number of language features used in tests:
> * Never iterating an iterable, always converting it
>   using `.toList()` first and iterating using indices
>   (fx `setEquals`).
>   Also require a `List` in places where an `Iterable`
>   wasn't necessary.
> * Avoid doing complicated computations that are also
>   used for the error message. Do simple check first,
>   then recompute to get better error messages
>   (fx `allDistinct`).
>
> Renamed some rarely used members for consistency
> (`stringContainsInOrder`->`containsInOrder`,
> where other string-contains functions just start
> with `contains`, and `containsOneOf` -> `containsAny`
> to match `Iterable.any` phrasing, and also it accepts
> if containing at least one, not precisely one.)
>
> Removed a function that wasn't used anywhere.
>
> Moved `assertStatementsEnabled` to `variations.dart` as `asserts`.
> Removed `typeAssertionsEnabled` and `checkedModeEnabled`. The former used in one place, where it was replaced with `checkedImplicitDowncasts` from `variations.dart`, the latter wasn't used anywhere.
>
> Deprecates `package:expect/minitest.dart`. It was never intended
> to be used for new tests, only as a help to convert existing tests
> written against `package:unit_test`.
> All existing imports marked as `// ignore: deprecated_member_use`.
>
> Change-Id: I07e21d4c0f3ccf11b82ee34af2668fdbb22264d2
> Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/352360
> Reviewed-by: Slava Egorov <vegorov@google.com>
> Reviewed-by: Ömer Ağacan <omersa@google.com>
> Reviewed-by: Nate Bosch <nbosch@google.com>
> Reviewed-by: Stephen Adams <sra@google.com>
> Commit-Queue: Lasse Nielsen <lrn@google.com>

Change-Id: I360b4347470a0bb2b63c3108e2b83ee2a771bf3f
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/362020
Reviewed-by: Nate Bosch <nbosch@google.com>
Reviewed-by: Ömer Ağacan <omersa@google.com>
Reviewed-by: Stephen Adams <sra@google.com>
Reviewed-by: Leaf Petersen <leafp@google.com>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Commit-Queue: William Hesse <whesse@google.com>

https://dart.googlesource.com/sdk/+/cd2c566bcf2263e93bef877443a784f3afe35549