Allow a `null` result (dart-lang/json_rpc_2#77)

Fixes dart-lang/json_rpc_2#76

Add missing nullable annotation on the `result` variable and a test
which fails if the variable is non-nullable.
3 files changed
tree: fb39f5bbad7c8ca8de59d47e6a0aa37afa3ea24f
  1. pkgs/