Dart pub update failed
Based on the comment discussion, this issue is due to you being behind a HTTP Proxy server. The Dart Editor and Dart Pub do no honor a system's proxy settings and only work if you have a normal connection out to the web. There are currently 2 bugs written up that the Dart team will hopefully fix at some point, but they are not there yet.
Please see more details about this issue on the bugs and star the bugs if you would like it to be fixed.
- Bug 5455: Allow Dart Editor to use a proxy for updates
- Bug 5454: Allow Pub to use an HTTP Proxy