add --preview flag to publish command

utils/pub/command_lish.dart

 // Copyright (c) 2012, 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.
 
 library command_lish;
 
 import 'dart:async';
 import 'dart:io';
 import 'dart:json';
 import 'dart:uri';
@@ skipping 14 matching lines @@
 /// Handles the `lish` and `publish` pub commands.
 class LishCommand extends PubCommand {
   final description = "Publish the current package to pub.dartlang.org.";
   final usage = "pub publish [options]";
   final aliases = const ["lish", "lush"];
 
   ArgParser get commandParser {
     var parser = new ArgParser();
     // TODO(nweiz): Use HostedSource.defaultUrl as the default value once we use
     // dart:io for HTTPS requests.
+    parser.addFlag('dry-run', abbr: 'n', 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;
   }
 
   /// The URL of the server to which to upload the package.
   Uri get server => Uri.parse(commandOptions['server']);
 
   Future _publish(packageBytes) {
     var cloudStorageUrl;
@@ skipping 106 matching lines @@
       var errors = pair.first;
       var warnings = pair.last;
 
       if (!errors.isEmpty) {
         throw "Sorry, your package is missing "
             "${(errors.length > 1) ? 'some requirements' : 'a requirement'} "
             "and can't be published yet.\nFor more information, see: "
             "http://pub.dartlang.org/doc/pub-lish.html.\n";
       }
 
+      if (commandOptions['dry-run']){
+        var s = warnings.length == 1 ? '' : 's';
+        log.warning("Package has ${warnings.length} warning$s.");
+        exit(0);

[Bob Nystrom, 2013/02/11 18:44:37]

We're trying to move away from explicitly calling exit() since that will do bad
things at some point in the future when dart:io makes stdin/stdout async.

Instead, can you make _validate return a Future<bool> and complete to true if
the publish should proceed and false if it shouldn't?

Then onRun can just check that and not call _publish if it's false.

[keertip, 2013/02/11 23:57:31]

Done.

+      }
+      
       var message = 'Looks great! Are you ready to upload your package';
 
       if (!warnings.isEmpty) {
         var s = warnings.length == 1 ? '' : 's';
         message = "Package has ${warnings.length} warning$s. Upload anyway";
       }
 
       return confirm(message).then((confirmed) {
         if (!confirmed) throw "Package upload canceled.";
       });
     });
   }
 }