[vm/ffi] asTypedList for Pointer<T extends AbiSpecificInteger>

[dcharkes]

Once #44589 is addressed, there is a question about how to provide a Dart API to users which has a size_t* in C.

For uint32_t*, the Dart API can be Uint32List. But for a type that is architecture dependent, we require runtime polymorphism at the moment. We could make the API return List<int> and take List<int> and try to cast, but polymorphism makes code slow.

We probably also don't want to introduce a SizeList, IntptrList, UintptrList to dart:typed_data. We introduced AbiSpecificInteger to dart:ffi to avoid hardcoding every possible type. Would it be possible to introduce a AbiSpecificTypedData<AbiSpecificInteger> to dart:typed_data or dart:ffi maybe?

Use case in ICU4X:

 class SizeList extends ffi.Struct {
  external ffi.Pointer<ffi.Size> _bytes;

  @ffi.Size()
  external int _length;

  // ignore: unused_element
  SizeList get _asDart => this;  // Return type Uint32List or Uint64List from dart:typed_data.

https://github.com/unicode-org/icu4x/blob/1c0b95ef95d869841fec45ed0e7b107b47c8afad/ffi/capi/dart/package/lib/src/lib.g.dart#L137-L144

Thanks @robertbastian for reporting!

Any thoughts @mkustermann @mraleph @lrhn ?