No need to type restrict io method

src/spectator/formatting/progress_formatter.cr

@@ -14,7 +14,7 @@ module Spectator::Formatting
     @skip_char : Colorize::Object(Char) = '*'.colorize(:yellow)
 
     # Output stream to write results to.
-    private getter io : IO
+    private getter io
 
     # Creates the formatter.
     def initialize(@io : IO = STDOUT)

src/spectator/formatting/summary.cr

@@ -9,7 +9,7 @@ module Spectator::Formatting
   # Classes including this module must implement `#io`.
   module Summary
     # Stream to write results to.
-    private abstract def io : IO
+    private abstract def io
 
     def start_dump
       io.puts