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>