[js_interop] should interpret a set of named parameters in dart as the last parameter in JS

[Cat-sushi]

// dart
external f(double a, {bool? b, bool? c});

f(1, c:true);

should be interpreted as

// JS
f(1, {c:true});

Because I think there are so many annoying XxOptions extension types in package:web.

[Cat-sushi]

I believe that the syntax of named parameters in dart was inspired by object literal in JS for options.
So, I think it is the time to take the advantage of the similarity of the syntax.

[srujzs]

Cool idea! I can see this being useful if you wanted to add different/additional fields that the XxOptions types may not support.

There are some shortcomings, however:

• Dart doesn't allow you to combine named and optional parameters. So if an options object itself is optional, we'll need a separate type.
• (As you already note) This would require the options parameter to always be last in the interop API.
• Since it doesn't use a type, it's only useful for passing in a value to an interop API and if users wanted to read the fields of such an object, they'll need a separate type that they can cast to.

[Cat-sushi]

• Dart doesn't allow you to combine named and optional parameters. So if an options object itself is optional, we'll need a separate type.

cf. dart-lang/language#1076