Add support for analyzer strong-mode

[TristanPct]

The generated code has errors if analysis_options.yaml is set with:

analyzer:
  strong-mode:
    implicit-casts: false
    implicit-dynamic: false

In generated code:

  @override
  PropertyMap<Driver> props(Driver instance) {
    return PropertyMap<Driver>({'name': instance.name});
  }

throws a Missing type arguments for map literal exception on the argument map.

Also:

  @override
  ClassValidator getClassValidator() {
    return ClassValidator<Car>(validators: [
      FieldMatchValidator(
          baseField: 'frontWheelCoverLeft', matchField: 'frontWheelCoverRight')
        ..affectedFields = ['frontWheelCoverLeft', 'frontWheelCoverRight']
        ..baseFieldMessage = carFieldMatchBaseFieldMessage,
      FieldMatchValidator(
          baseField: 'rearWheelCoverLeft', matchField: 'rearWheelCoverRight')
        ..affectedFields = ['rearWheelCoverLeft', 'rearWheelCoverRight']
        ..matchFieldMessage = carFieldMatchMatchFieldMessage
    ]);
  }

throws a Missing type arguments for generic type 'FieldMatchValidator' exception on FieldMatchValidator.