Make simple_printer.dart export optional

Hi! Importing dart:io as a dependency makes the package incompatible with browser projects.

One solution can be making simple_printer.dart export optional like file_output.dart

See: simc/logger@9d64611#diff-9efc0241babc652e668a9441d2926a03R2

Author: marcgraub

lib/logger.dart

@@ -9,12 +9,14 @@ export 'src/outputs/console_output.dart';
 export 'src/outputs/stream_output.dart';
 export 'src/outputs/memory_output.dart';
 export 'src/printers/pretty_printer.dart';
-export 'src/printers/simple_printer.dart';
 export 'src/printers/logfmt_printer.dart';
 
 export 'src/log_output.dart'
     if (dart.library.io) 'src/outputs/file_output.dart';
 
+export 'src/log_printer.dart'
+    if (dart.library.io) 'src/printers/simple_printer.dart';
+
 export 'src/log_filter.dart';
 export 'src/log_output.dart';
 export 'src/log_printer.dart';