Support new constructor syntax.

[davidmorgan]

Reject primary constructors cleanly; we can look at adding support for them properly as a bigger feature.

[davidmorgan]

@donny-dont I think this is ready now--please take a look and see if it does what you need :)

[vanyasem]

I can confirm that this fixes my issue with with new constructors. built_value no longer throws an error, and generates cleanly
Tested on both BuiltValue and EnumClass

[donny-dont]

@donny-dont I think this is ready now--please take a look and see if it does what you need :)

I just added the following lint rules

    - empty_container_bodies
    - initialize_in_field_declaration
    - unnecessary_const_in_enum_constructor
    - unnecessary_primary_constructor_body
    - unnecessary_type_name_in_constructor
    - use_declaring_parameters

Applied them through dart fix --apply . and then ran dart run build_runner build successfully so this is good to go.

Thanks @davidmorgan !