code: private void alwaysAssert(boolean expression, String msg,
        Object... formatArgs) {
      if (!expression) {
        throw new AssertionError(String.format(msg, formatArgs));
      }
    }
docs: @throws AssertionError if the given expression is not true.
  The message of the exception is formatted using the 'msg' and
  'formatArgs' parameters.
  
  This should be used in preference to Java's built-in assert in
  non-performance-critical paths, where a failure of this invariant
  might cause the protocol to lose data.