sendCommand method Null safety

Implementation

Future<dynamic> sendCommand(String script, Duration? duration) async {
  // This code should not be reachable before the VM service extension is
  // initialized. The VM service extension is expected to initialize both
  // `$flutterDriverResult` and `$flutterDriver` variables before attempting
  // to send commands. This part checks that `$flutterDriverResult` is present.
  // `$flutterDriver` is not checked because it is covered by the `script`
  // that's executed next.
  try {
    await _driver.execute(r'return $flutterDriverResult', <String>[]);
  } catch (error, stackTrace) {
    throw DriverError(
      'Driver extension has not been initialized correctly.\n'
      'If the test uses a custom VM service extension, make sure it conforms '
      'to the protocol used by package:integration_test and '
      'package:flutter_driver.\n'
      'If the test uses VM service extensions provided by the Flutter SDK, '
      'then this error is likely caused by a bug in Flutter. Please report it '
      'by filing a bug on GitHub:\n'
      '  https://github.com/flutter/flutter/issues/new?template=2_bug.md',
      error,
      stackTrace,
    );
  }

  String phase = 'executing';
  try {
    // Execute the script, which should leave the result in the `$flutterDriverResult` global variable.
    await _driver.execute(script, <void>[]);

    // Read the result.
    phase = 'reading';
    final dynamic result = await waitFor<dynamic>(
      () => _driver.execute(r'return $flutterDriverResult', <String>[]),
      matcher: isNotNull,
      timeout: duration ?? const Duration(days: 30),
    );

    // Reset the result to null to avoid polluting the results of future commands.
    phase = 'resetting';
    await _driver.execute(r'$flutterDriverResult = null', <void>[]);
    return result;
  } catch (error, stackTrace) {
    throw DriverError(
      'Error while $phase FlutterDriver result for command: $script',
      error,
      stackTrace,
    );
  }
}