Removing wrapper.c

[dcharkes]

We currently need wrapper.c because dart:ffi does not support passing structs by value yet (#540).

However, we also have other things in wrapper.c, for example having some process uuid and a stack.

We should investigate whether we can remove those things and do it in Dart. Or document why it is not possible to do it in only Dart + dart:ffi. (That might tell us we need more features in dart:ffi.)

Also, we could make a branch for this package which works on the locally built https://dart-review.googlesource.com/c/sdk/+/140290, so that we can merge that brach after structs-by-value lands and is released in stable.

[dcharkes]

After some digging and discussion:

if
clang.clang_visitChildren_wrap_root only calls _rootCursorVisitor and
clang.clang_visitChildren_wrap_structMembers only calls _structMembersVisitor and
clang.clang_visitChildren_wrap_enum only calls _enumCursorVisitor and
clang.clang_visitChildren_wrap_typedefdeclaration only calls _typedefdeclarationCursorVisitor and
(etc.)
you keep a stack in Dart with what Structs you expect the next callbacks (for struct members) to be for,
you don't need a stack in C.

You still need a mapping from IsolateUUID to _rootCursorVisitor etc.
dlopen only loads the dynamic library once into memory if called multiple times.
And, package:test runs tests concurrently in the same VM.
When we get support for structs by value, this issue goes away, because you would pass the callback function pointer of the function in the specific isolate.

[dcharkes]

We could consider removing support for running tests concurrently (and removing the isolate uuid) by using --concurrency=1, if that does not make the tests run overly long (and we can somehow make it always do that without users having to specify the flag).

[mannprerak2]

Done in dart-archive/ffigen#164.