[js_interop] No error if a local variable or function is annotated with `JSExport()`

[sgrekhov]

No expected errors in the test below on dart2js.

import 'dart:js_interop';

void main() {
  @JSExport()
  int variable = 42;
//    ^^^^^^^^
// [analyzer] unspecified
// [web] unspecified

  @JSExport()
  void func() {}
//     ^^^^
// [analyzer] unspecified
// [web] unspecified
}

cc @srujzs

Dart SDK version: 3.9.0-edge.55523e53df6f5ec1e9dad382d370cc8fd7262aeb (main) (Thu Jul 10 00:02:13 2025 -0700) on "linux_x64"

UPD. The test above passes on dart2wasm but fails on dart2js. Probably it is the same test.py issue as #61084

[sgrekhov]

Also no error for an old-fashioned function type.

import 'dart:js_interop';

@JSExport()
typedef void Foo(); // No expected error here
//           ^^^
// [analyzer] unspecified
// [web] unspecified

void main() {
  print(Foo);
}