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