Documentation of Uint8List.operator + is incorrect. It returns a List<int>.

[whesse]

The documentation of Uint8List.operator + says that if both operands are Uint8List, it returns a Uint8List.
This is incorrect, it always returns a List.

The documentation says:

"Returns a concatenation of this list and other.

If other is also a typed-data list, then the return list will be a typed data list
capable of holding both unsigned 8-bit integers 
and the elements of other, otherwise it'll be a normal list of integers."

That this means it can return a Uint8List is made clear by the note in the documentation of List.operator +, that says:

"Returns a new list containing the elements of this list followed by the elements of other.

The default behavior is to return a normal growable list. Some list types
may choose to return a list of the same type as 
themselves (see [Uint8List.+](https://api.dart.dev/stable/2.18.7/dart-typed_data/Uint8List/operator_plus.html));

From the beginning, when this method was added to Uint8List in https://dart-review.googlesource.com/c/sdk/+/39140
, the method did not return a Uint8List, but inherited its implementation through _TypedIntList, https://dart-review.googlesource.com/c/sdk/+/39140/4/sdk/lib/typed_data/typed_data.dart#386 , which has since been replaced by a mixin.

I think the implementation of these classes should be changed so that this documentation is true, but until then, the
documentation should be correct.

This override of Uint8List.operator + is also not present on Int8List or other similar classes. Only Uint8 has it.