[js_interop] No error or warning when typedef is annotated with the `@anonymous`

[sgrekhov]

Now the folllwong test is passing, no error or warning occurs

import 'dart:js_interop';
import 'dart:js_interop_unsafe';
import '../../../Utils/expect.dart';
import '../js_utils.dart';

String log = "";

@JSExport()
class C {
  int variable = 42;
  String method(String v) => "method($v);";
  String get getter => "Some getter";
  void set setter(bool value) {
    log = "setter($value);";
  }
}

@JSExport()
typedef CAlias = C;

void main() {
  var c = CAlias();
  var jsC = createJSInteropWrapper<CAlias>(c);
  globalContext["jsC"] = jsC;
  eval(r'''
    globalThis.v1 = globalThis.jsC.variable;
    globalThis.v2 = globalThis.jsC.method('x');
    globalThis.v3 = globalThis.jsC.getter;
    globalThis.jsC.setter = false;
  ''');
  Expect.equals(42, (globalContext["v1"] as JSNumber).toDartInt);
  Expect.equals("method(x);", (globalContext["v2"] as JSString).toDart);
  Expect.equals("Some getter", (globalContext["v3"] as JSString).toDart);
  Expect.equals("setter(false);", log);
}

It makes sense to expect a warning on typedef annotaded with @anonymous if it is technically possible.

cc @srujzs @eernstg @johnniwinther

[eernstg]

Sounds good, thanks! If it turns out to be impractical for backends to have any opinion about type aliases (because they have been compiled away as the Kernel code was generated), it might still be possible to do other things. By the way, it would be relevant to @anonymous just as well as @JSExport(...).

For example (admittedly, a long shot), @pragma('metadata-for:class,enum,member') on the declaration of JSExport could allow the analyzer to report that @JSExport() isn't intended to bu used on a type alias.