code: void expectStringOrSymbol(Node n, JSType type, String msg) {
      if (!type.matchesStringContext() && !type.matchesSymbolContext()) {
        mismatch(n, msg, type, STRING_SYMBOL);
      }
    }
docs: Expect the type to be a string or symbol, or a type convertible to a string. If the expectation
  is not met, issue a warning at the provided node's source code position.