function assertIsButton(event, button, isActionButton) {
  for (let key in Button) {
    assertEquals(
        'Testing isButton(' + key + ') against ' + button + ' and type ' +
            event.type,
        Button[key] == button, event.isButton(Button[key]));
  }

  assertEquals(isActionButton, event.isMouseActionButton());
}
@param {!BrowserEvent} event
@param {BrowserEvent.MouseButton} button
@param {boolean} isActionButton
@return {!BrowserEvent}
@suppress {missingReturn} suppression added to enable type checking