Make stdout and stderr async.

sdk/lib/_internal/pub/bin/pub.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:async';
 import 'dart:io';
 import 'dart:math' as math;
 
 import 'package:args/args.dart';
 import 'package:path/path.dart' as path;
@@ skipping 30 matching lines @@
   }
 
   if (options.command == null) {
     if (options.rest.isEmpty) {
       // No command was chosen.
       printUsage();
     } else {
       log.error('Could not find a command named "${options.rest[0]}".');
       log.error('Run "pub help" to see available commands.');
       flushThenExit(exit_codes.USAGE);
-      return;
     }
     return;
   }
 
   if (options['trace']) {
     log.recordTranscript();
   }
 
   switch (options['verbosity']) {
     case 'normal': log.showNormal(); break;
@@ skipping 104 matching lines @@
   for (var name in names) {
     buffer.write('  ${padRight(name, length)}   '
         '${PubCommand.commands[name].description}\n');
   }
 
   buffer.write('\n');
   buffer.write(
       'Use "pub help [command]" for more information about a command.');
   log.message(buffer.toString());
 }