Deprecate TypeSafeWrappers? about collection

[nex3]

As long as we're only supporting Dart 2 (which appears to be the case based on the current pubspec), we can do this much more simply by just using cast() everywhere and deleting the TypeSafe wrappers.

[matanlurey]

Definitely, I just am trying to be nice to users - in my own tests it looks like the type of errors thrown by Dart2 versus thrown currently by the wrappers are not exactly the same.

[nex3]

How much does that buy us in practice? I don't see how code could ever actually construct a TypeSafeList with your proposed intermediate implementation.

[matanlurey]

Until the VM and Dart2JS enforce strong mode, strong-mode typed lists will not throw. So you could potentially be using .typed for safety, and it throws in DDC but not Dart2JS.

[nex3]

But if dart2js isn't using strong mode semantics, then base is List<E> will always be true (because <E> is erased) and nothing will throw anyway.