add --preview flag to publish command

utils/pub/command_lish.dart

Index: utils/pub/command_lish.dart
===================================================================
--- utils/pub/command_lish.dart	(revision 18263)
+++ utils/pub/command_lish.dart	(working copy)
@@ -32,6 +32,8 @@
     var parser = new ArgParser();
     // TODO(nweiz): Use HostedSource.defaultUrl as the default value once we use
     // dart:io for HTTPS requests.
+    parser.addFlag('preview', abbr: 'p', negatable: false,
+        help: 'Preview publishing the package');
     parser.addOption('server', defaultsTo: 'https://pub.dartlang.org',
         help: 'The package server to which to upload this package');
     return parser;
@@ -158,6 +160,12 @@
             "http://pub.dartlang.org/doc/pub-lish.html.\n";
       }
 
+      if (commandOptions['preview']){
+        var s = warnings.length == 1 ? '' : 's';
+        log.warning("Package has ${warnings.length} warning$s.");
+        exit(0);
+      }
+      
       var message = 'Looks great! Are you ready to upload your package';
 
       if (!warnings.isEmpty) {