[dart:js_interop] Support allowInteropCaptureThis in some manner

[srujzs]

Previous iterations of JS interop allowed wrapping callbacks that allowed you to capture the calling this object.

sdk/sdk/lib/js_util/js_util.dart

Line 281 in b7aa910

external Function allowInteropCaptureThis(Function f);

While we do have support for allowInterop through Function.toJS, we don't support capturing this. Currently, that js_util method is unsupported in dart2wasm and the usage is low, but it would be nice to support the capture through something like .toJSWithThis so users can migrate off of dart:js_util easier.

[ykmnkmi]

Not necessary, but I need this.

[srujzs]

FYI @nex3, iirc, Sass was one of the few codebases using this. There might be a way to work around this in the meantime, but I haven't gotten a chance to dig deeper. Let me know if this is a blocker.

[nex3]

Yes, this is necessary. Generally speaking, JS libraries treat this as part of the API surface area for callbacks.

[Zekfad]

Currently it is possible to partially workaround this by using raw JS wrapper function, although this will fail in contexts where CSP does not allow unsafe-eval.

@JS('Function')
external JSFunction _createDynamicJSFunctionUnsafe$1(JSString argument, JSString body);

final _captureThis = () {
  try {
    return _createDynamicJSFunctionUnsafe$1(
      'f'.toJS,
      'return function (...args) { return f.call(this, this, ...args); };'.toJS,
    );
  } catch(e) {
    return null;
  }
}();

JSFunction _tryCaptureThis$0<R extends JSAny?>(R Function(JSObject? context) f) {
  final jsF = f.toJS;
  if (_captureThis == null) {
    return (() => f(null)).toJS;
  }
  return _captureThis!.callAsFunction(_captureThis, jsF)! as JSFunction;
}

JSFunction _tryCaptureThis$1<R extends JSAny?, Arg1 extends JSAny?>(R Function(JSObject? context, Arg1 arg1) f) {
  final jsF = f.toJS;
  if (_captureThis == null) {
    return ((Arg1 arg1) => f(null, arg1)).toJS;
  }
  return _captureThis!.callAsFunction(_captureThis, jsF)! as JSFunction;
}