Upgraded analyzer to 4.1.0

[marcjoha]

...and cleaned up some warnings.

[marcjoha]

If anyone wants to use this before it's merged, replace the current dev dependency like below. The regular dependency definition can be kept as is.

dev_dependencies:
  flutter_sheet_localization_generator:
    git:
      url: https://github.com/marcjoha/flutter_sheet_localization.git
      path: flutter_sheet_localization_generator

[Blikun]

thanks