add --preview flag to publish command

utils/tests/pub/pub_test.dart

Index: utils/tests/pub/pub_test.dart
===================================================================
--- utils/tests/pub/pub_test.dart	(revision 18320)
+++ utils/tests/pub/pub_test.dart	(working copy)
@@ -105,8 +105,11 @@
             Publish the current package to pub.dartlang.org.
 
             Usage: pub publish [options]
-            --server    The package server to which to upload this package
-                        (defaults to "https://pub.dartlang.org")
+            -n, --dry-run    Preview publishing the package
+                             Validate but do not publish the package
+
+                --server     The package server to which to upload this package
+                             (defaults to "https://pub.dartlang.org")
             ''');
     });