Handle ObjC nullable annotations

[liamappelbe]

Also, make ObjCNullable a proper type, rather than just a nullability flag on objc methods.

Fixes dart-lang/native#262

Example (from nullable_test):

@interface NullableInterface  : NSObject {
}

+(BOOL) isNullWithNullableNSObjectArg:(nullable NSObject *)x;
+(BOOL) isNullWithNotNullableNSObjectPtrArg:(NSObject *)x;
+(nullable NSObject *) returnNil:(BOOL)r;

@property (nullable, retain) NSObject *nullableObjectProperty;

@end

Generates:

class NullableInterface extends NSObject {
  NullableInterface._(ffi.Pointer<ObjCObject> id, NullableTestObjCLibrary lib,
      {bool retain = false, bool release = false})
      : super._(id, lib, retain: retain, release: release);

  /// Returns a [NullableInterface] that points to the same underlying object as [other].
  static NullableInterface castFrom<T extends _ObjCWrapper>(T other) {
    return NullableInterface._(other._id, other._lib,
        retain: true, release: true);
  }

  /// Returns a [NullableInterface] that wraps the given raw object pointer.
  static NullableInterface castFromPointer(
      NullableTestObjCLibrary lib, ffi.Pointer<ObjCObject> other,
      {bool retain = false, bool release = false}) {
    return NullableInterface._(other, lib, retain: retain, release: release);
  }

  /// Returns whether [obj] is an instance of [NullableInterface].
  static bool isInstance(_ObjCWrapper obj) {
    return obj._lib._objc_msgSend_0(obj._id, obj._lib._sel_isKindOfClass_1,
        obj._lib._class_NullableInterface1);
  }

  static bool isNullWithNullableNSObjectArg_(
      NullableTestObjCLibrary _lib, NSObject? x) {
    return _lib._objc_msgSend_22(_lib._class_NullableInterface1,
        _lib._sel_isNullWithNullableNSObjectArg_1, x?._id ?? ffi.nullptr);
  }

  static bool isNullWithNotNullableNSObjectPtrArg_(
      NullableTestObjCLibrary _lib, NSObject x) {
    return _lib._objc_msgSend_23(_lib._class_NullableInterface1,
        _lib._sel_isNullWithNotNullableNSObjectPtrArg_1, x._id);
  }

  static NSObject? returnNil_(NullableTestObjCLibrary _lib, bool r) {
    final _ret = _lib._objc_msgSend_24(
        _lib._class_NullableInterface1, _lib._sel_returnNil_1, r);
    return _ret.address == 0
        ? null
        : NSObject._(_ret, _lib, retain: true, release: true);
  }

  NSObject? get nullableObjectProperty {
    final _ret = _lib._objc_msgSend_25(_id, _lib._sel_nullableObjectProperty1);
    return _ret.address == 0
        ? null
        : NSObject._(_ret, _lib, retain: true, release: true);
  }

  set nullableObjectProperty(NSObject? value) {
    return _lib._objc_msgSend_26(
        _id, _lib._sel_setNullableObjectProperty_1, value?._id ?? ffi.nullptr);
  }

  @override
  NullableInterface init() {
    final _ret = _lib._objc_msgSend_2(_id, _lib._sel_init1);
    return NullableInterface._(_ret, _lib, retain: true, release: true);
  }

  static NullableInterface new1(NullableTestObjCLibrary _lib) {
    final _ret =
        _lib._objc_msgSend_2(_lib._class_NullableInterface1, _lib._sel_new1);
    return NullableInterface._(_ret, _lib, retain: false, release: true);
  }

  static NullableInterface allocWithZone_(
      NullableTestObjCLibrary _lib, ffi.Pointer<_NSZone> zone) {
    final _ret = _lib._objc_msgSend_3(
        _lib._class_NullableInterface1, _lib._sel_allocWithZone_1, zone);
    return NullableInterface._(_ret, _lib, retain: false, release: true);
  }

  static NullableInterface alloc(NullableTestObjCLibrary _lib) {
    final _ret =
        _lib._objc_msgSend_2(_lib._class_NullableInterface1, _lib._sel_alloc1);
    return NullableInterface._(_ret, _lib, retain: false, release: true);
  }
}

Diff:

32,34c32,34
<       NullableTestObjCLibrary _lib, NSObject? x) {
<     return _lib._objc_msgSend_22(_lib._class_NullableInterface1,
<         _lib._sel_isNullWithNotNullableNSObjectPtrArg_1, x?._id ?? ffi.nullptr);
---
>       NullableTestObjCLibrary _lib, NSObject x) {
>     return _lib._objc_msgSend_23(_lib._class_NullableInterface1,
>         _lib._sel_isNullWithNotNullableNSObjectPtrArg_1, x._id);
37,38c37,38
<   static NSObject returnNil_(NullableTestObjCLibrary _lib, bool r) {
---
>   static NSObject? returnNil_(NullableTestObjCLibrary _lib, bool r) {
40c40,42
<     return NSObject._(_ret, _lib, retain: true, release: true);
---
>     return _ret.address == 0
>         ? null
>         : NSObject._(_ret, _lib, retain: true, release: true);

[dcharkes]

The CI is still red, is this ready for review or WIP?

[liamappelbe]

The CI is still red, is this ready for review or WIP?

Ready for review. The fixes to these tests should be pretty minor. I'm working on them now.

[dcharkes]

lgtm

nit: now that we don't check in generated files anymore, please add a pre-and-post FFIgen output of a small example to the PR description so it's easy to see the diff.

[dcharkes]

Nice, much cleaner!