PlatformDropdownButton is not formatted correctly

[jaredsburrows]

PlatformDropdownButton is not showing the correct theme and style on iOS:

Example:
dart

                    PlatformDropdownButton<String>(
                        value: _indent,
                        onChanged: (String? newValue) {
                          if (newValue != null) {
                            setState(() {
                              _indent = newValue;
                              _formatOrFlattenJson(_codeController.text);
                            });
                          }
                        },
                        items: const <DropdownMenuItem<String>>[
                          DropdownMenuItem<String>(
                            value: '  ',
                            child: Text('2 spaces'),
                          ),
                          DropdownMenuItem<String>(
                            value: '    ',
                            child: Text('4 spaces'),
                          ),
                        ],
                      ),