Allow transfer of Pointer objects across isolates

[mkustermann]

Our Pointer objects are no longer reified. We won't allow attaching finalizers to them. We may just represent them as unboxed integers in Dart.

Given that, it does seem reasonable we allow sending Pointer objects across isolates (since they are immutable, they can also be shared). If the message that is being sent includes a Finalizable we disallow sending the message anyway.

Currently people can still achieve it by taking pointer.address before sending and then re-construct the pointer on the other side.

(There's one caveat: If a message got sent but receiver dies or doesn't handle them, any messages are dropped. This dropping will not cause freeing of any memory (we have support on C side to do that, but not in SendPort api). But this problem seems orthogonal.)

/cc @mraleph @dcharkes

[dcharkes]

Duplicate of:

• [ffi/isolate] Support passing/receiving Pointers to/from Isolates. #50457