See #2486 and #2502. If the Pub website is blocked, the API check for versions will also fail, and we'll never update.

Some possible options:

• Call the API on pub.flutter-io.cn (if supported) if the first request fails
• Always try to update DevTools in the background at startup (note: this might mess with people using locally-activated versions)
• Periodically prompt the user to update ("Unable to check for updates, install latest version?" or similar)