lint: Enable always_declare_return_types

Otherwise the syntax for a function declaration looks a lot like
that for a function call.

Author: gnprice

analysis_options.yaml

@@ -21,6 +21,7 @@ linter:
   # For a list of all available lints, with docs, see:
   #   https://dart-lang.github.io/linter/lints/index.html.
   rules:
+    always_declare_return_types: true
     no_literal_bool_comparisons: true
     prefer_relative_imports: true
     unnecessary_statements: true