Make pub publish more user friendly:

- Show the contents of their package.
- Let the confirm the upload even if there are no warnings.
- Make the error/warning text less scary.
- Validate that the pubspec has a version client-side.

Addresses #7175 and #7219.

Review URL: https://codereview.chromium.org//11557008

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@16068 260f80e4-7a28-3924-810f-c04153c831b5