docs: public final JsExpr assertExpr() {
      RequiresCollector.IntoImmutableSet collector = new RequiresCollector.IntoImmutableSet();
      JsExpr expr = assertExprAndCollectRequires(collector);
      ImmutableSet<GoogRequire> requires = collector.get();
      if (!requires.isEmpty()) {
        throw new IllegalStateException("calling assertExpr() would drop requires!: " + requires);
      }
      return expr;
    }