This PR adds the following lint rules to our list:
```
always_put_required_named_parameters_first
avoid_multiple_declarations_per_line
avoid_positional_boolean_parameters
avoid_returning_null_for_void
avoid_returning_this
avoid_unnecessary_containers
enable_null_safety
library_private_types_in_public_api
no_leading_underscores_for_library_prefixes
no_leading_underscores_for_local_identifiers
prefer_null_aware_method_calls
tighten_type_of_initializing_formals
unnecessary_late
use_setters_to_change_properties
```
And these rules were considered, and some changes were made according to
them as a clean-up, but in many places they didn't make sense
(`prefer_asserts_with_message` I would have included, but there were too
many places that needed to be changes):
```
collection_methods_unrelated_type
prefer_asserts_with_message
avoid_renaming_method_parameters
```
