The type variable `T`, when used as an expression, will always be a
`Type`. The type test `T is! List` is always true (as is `T is Type`).

This expression will become a warning in the analyzer in some upcoming
release of the Dart SDK.

This `assert` was added in a PR which reapplied an earlier PR, however
the earlier PR did not include this assert. I do not see any discussion
indicating the intent of this assert.

The impact of testing this `T` in any way is low - this is a private
class so we can see all the uses and know that the only type bound to
`T` is `Uri`. Avoid the upcoming diagnostic and remove the assert
entirely. This maintains existing behavior but ignores the potential
intent for the check.