openDevTools method
Opens a DevTools window for the target.
targetId This can be the page or tab target ID.
Returns: The targetId of DevTools page target.
Implementation
Future<TargetID> openDevTools(TargetID targetId) async {
  var result = await _client.send('Target.openDevTools', {
    'targetId': targetId,
  });
  return TargetID.fromJson(result['targetId'] as String);
}