Provide convenience conversion for Map->NSDictionary and List->NSArray

[stuartmorgan-g]

Objective-C has very convenient literal syntax for arrays and dictionaries, much like Dart does. With those not available in Dart, it looks like the only way to make an array or dictionary right now is either dynamically via adding objects, or with the old-style methods like initWithObjectsAndKeys: and initWithObjects, which are clunky and error prone. (And actually, it doesn't look like those specific ones currently work due to #1192.)

It would be very helpful to be able to initialize them from a List or Map, the way NSString has a convenience constructor that takes a String.

[stuartmorgan-g]

In the meantime I tossed together a partial version for my own use:

NSObject? _convertKnownType(Object? o) {
  return switch (o) {
    null => null,
    final String s => NSString(s),
    final List<Object?> l => _convertList(l),
    final Map<Object?, Object?> m => _convertMap(m),
    final int i => NSNumber.numberWithInt_(i),
    final double d => NSNumber.numberWithDouble_(d),
    _ => throw UnimplementedError('No conversion for $o'),
  };
}

NSObject _covertKnownTypeWithNSNull(Object? o) {
  return _convertKnownType(o) ?? NSNull.null1();
}

NSArray _convertList(List<Object?> list) {
  final NSMutableArray array = NSMutableArray.arrayWithCapacity_(list.length);
  for (final Object? o in list) {
    array.addObject_(_covertKnownTypeWithNSNull(o));
  }
  return array;
}

NSDictionary _convertMap(Map<Object?, Object?> map) {
  final NSMutableDictionary dict =
      NSMutableDictionary.dictionaryWithCapacity_(map.length);
  for (final MapEntry<Object?, Object?> entry in map.entries) {
    dict.setObject_forKey_(_covertKnownTypeWithNSNull(entry.value),
        _covertKnownTypeWithNSNull(entry.key));
  }
  return dict;
}

Some things I would ideally want from a real version of this:

• _convertMap and _convertList to be exposed on NSDictionary and NSArray as factory constructors.
• Provide a convenience constructor for NSNumber #1445 for a simpler NSNumber conversion.
• An optional closure to handle arbitrary types, so that as a client I could pass in a collection with types known to me but unknown to the package, and have them converted per my own code wherever they were in the (arbitrarily nested) collection structure.
• Better construction using the variadic collection creation once Bindings for ObjC methods that have variadic arguments. #1192 is fixed, to cut down on the number of messages sent.