[vm/ffi] allocate free when adding finalizers in Dart

[dcharkes]

We have exposed native finalizers through dart:ffi (6544c69 and 7eac9f3), which enables users to write finalizers in C/C++.

We might still want to expose finalizers in Dart as well (#35770). However, on Windows looking up free in different dynamic libraries and the DartVM results in a different free that can only be called on memory allocated with the malloc in the corresponding dynamic library.

Therefore, we use a Windows API for allocating and freeing on Windows currently in package:ffi:

void free(Pointer pointer) {
  if (Platform.isWindows) {
    if (winHeapFree(processHeap, /*flags=*/ 0, pointer) == 0) {
      throw ArgumentError("Could not free $pointer.");
    }
  } else {
    posixFree(pointer);
  }
}

However, this function does not have the same signature as malloc and free.

We should consider having a free-symbol in the DartVM that uses mimics this behavior but just has the free-signature. (The same for allocate.)

And then it might make more sense to move allocate/free to dart:ffi instead of package:ffi.

[dcharkes]

We have worked around this by using CoTaskMemAlloc instead.

• dart-archive/ffi@6a9e991
• https://dart-review.googlesource.com/c/sdk/+/260760/6/runtime/bin/BUILD.gn