Fix newly enforced package:pedantic lints (#1118)

- always_declare_return_types
- annotate_overrides
- await_only_futures
- omit_local_variable_types
- prefer_collection_literals
- prefer_conditional_assignment
- prefer_final_fields
- prefer_for_elements_to_map_fromiterable
- prefer_if_null_operators
- prefer_spread_collections
- unnecessary_this
- use_function_type_syntax_for_parameters

Bump min SDK to 2.3.0 everywhere to allow using spreads in collection
literals.

Skip an already failing test