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