Dart: enable custom fields with defaults that do not provide const constructor to work with 'PositionalDefaults'

[pwrobeldev]

------ Motivation ------

When fields with default values were defined in a struct
and their types did not provide 'const' constructors, when
such fields were used in combination with 'PositionalDefaults',
then the generated code did not compile.

------ Designed solution ------

In order to be able to generate the code, which compiles
when 'PositionalDefaults' annotation is used together
with default values of types that do not provide const
constructors, the optional values have been used.

When Gluecodium encounters a type that does not
provide const constructor it makes it optional in
positional defaults constructor and uses null as
default.

Later, on initializer list it checks if user provided
value different than null and if not, then initializes
the field with the actual default value. This way
the limitation related to const constructors is surpassed.

However, if the user wants to have nullable type that
does not provide const constructor and uses default
value different than null, then the behavior may be surprising.
Therefore, the validator was implemented for Dart to prevent
users from such usage.

------ Contents of change ------

1. New smoke and functional tests, which confirm invalid behavior.
2. Implementation of designed fix.
3. New functional tests, which confirm that the fix works as expected.
4. Validator of default values to prevent unexpected behavior + unit tests.