docs: private void captureFailure(AssertionError captured) {
      if (failure != null) {
        // TODO(diamondm) is it worthwhile to add the failures as suppressed exceptions?
        throw new AssertionError(
            lenientFormat(
                "ExpectFailure.whenTesting() caught multiple failures:\n\n%s\n\n%s\n",
                Platform.getStackTraceAsString(failure), Platform.getStackTraceAsString(captured)));
      }
      failure = captured;
    }