[js_interop] Make it an error to declare an external constructor on an interop extension type whose core interop type is not <: JSObject

[sgrekhov]

The following code produces a run-time error on dart2js but works on dart2wasm.

import 'dart:js_interop';

@JS('Number')
extension type MyNumber._(JSNumber _) implements JSNumber {
  external MyNumber.fromDart(num value);
}

main() {
  // dart2wasm: prints 42
  // dart2js: TypeError: Instance of 'Number': type 'UnknownJavaScriptObject' is not a subtype of type 'double'
  print(MyNumber.fromDart(42));
}

Please note that according to the interop documentation MyNumber.fromDart(42) invokes constructor of JS Number using new. When this constructor is invoked with 'new' it returns "a wrapping Number object, which is not a primitive.". If the returned object is not a Number primitive then may be it's not a subtype of MyNumber representation type and this explains the error on dart2js? I don't know, it's just a suppose.

@srujzs @osa1 is this an issue or may be expected but undocumented behavior? What should be expected in this case?

Dart SDK version: 3.10.0-edge.f98e53cd40fe0abc7c7751b8146a0db9cea99758 (main) (Wed Aug 13 23:32:49 2025 -0700) on "linux_x64"

[srujzs]

If the returned object is not a Number primitive then may be it's not a subtype of MyNumber representation type and this explains the error on dart2js?

Your intuition is correct and it's expected, but we should make it an error to declare external constructors on extensions types whose core interop type is not a subtype JSObject or JSAny. They'll always give a value that will be incompatible with the JS type.