[ffi] Explore more concise dynamic library binding

[dcharkes]

Explore if we could use type annotations on dynamic library function signatures (similar to struct fields) to avoid writing two signatures.

Moreover, the asLibrary would dynamically link all functions in one go.

abstract class FooBar extends Library {
  @Int32
  int Truncate64(@Int64 int x);
}

// Part of dart:ffi
class DynamicLibrary {
  // ...
  T asLibrary<T extends Library>();
}

Suggested in #34452 (comment)

/cc @mraleph @sjindel-google

[dcharkes]

Annotations on typedefs? It's more likely than you think!
@eernst Erik, where are we with filling in the annotation holes in the grammar?

Originally posted by @lrhn in #34452 (comment)

@eernstg and @lrhn: Are typedefs inlined in the dart compiler? And if so, are annotations preserved?

typedef FunctionPointer = @Int32 int TruncationFunction(@Int64 int x);

Pointer<FunctionPointer> myPointer = //...

[lrhn]

Ah, I meant something like:

@Something
typedef MyType = int Function(int);

Putting annotations into the value is probably not going to work. Annoatations in Dart goes on declarations, in general, things that have a name or identity. Structural types are not such a thing.

[dcharkes]

Now that we have https://pub.dev/packages/ffigen, we might not need this as much as before.

[dcharkes]

We will address this by introducing assets:

• ☂️ [vm/ffi] Native assets feature #50565