1.0.0-nullsafety.0

Addressed the remaining TODOs, while improving the description of some decisions. Also fixing the bad version naming.

• Updated dart-lang/linter rules up to version 0.1.129: added rule avoid_dynamic_calls (experimental, still not
  enabled);
• Enabled avoid_annotating_with_dynamic - which was erroneously disabled (misconception of what it really did);
• Enabled strong-mode rule of implicit-dynamic to false;
• Enabled cast_nullable_to_non_nullable, tighten_type_of_initializing_formals, unnecessary_null_checks and
  unnecessary_nullable_for_final_variable_declarations due to NNBD;
• Disabled avoid_returning_null, avoid_returning_null_for_future and avoid_returning_null_for_void due to NNBD;
• Customized avoid_as to show a warning, with the according explanation to why;

Contains all rules up to version 0.1.127 of dart-lang/linter, with pending TODOs (non-critical) in the
analysis_options.yaml.