gistfile.java

@JsFunction
interface Interface {
  Interface invoke(String arg);
}

@JsType(isNative = true, namespace = JsPackage.GLOBAL)
class Exports {
  @JsProperty(namespace = JsPackage.GLOBAL)
  public static native void setInterface(Interface value);
}

@SuppressWarnings("Duplicates")
public class ExportInterface implements EntryPoint {
  @Override
  public void onModuleLoad() {
    JsGlobals.getWindow().getConsole().log("setting Interface");
    Interface value = new Interface() {
      @Override
      public Interface invoke(String arg) {
        JsGlobals.getWindow().getConsole().log(arg);
        return this;
      }
    };
    Exports.setInterface(value);
  }
}