"Building flutter tool..." takes forever

Question

Due to some Android Studio problem, I invalidated flutter cache.

When I try flutter upgrade, I'm stuck with the "Building flutter tool ..." phase

$ flutter upgrade
Downloading Dart SDK from Flutter engine beb8a7ec48f6b980a778d19eeda613635c3897c9...
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100    16  100    16    0     0     25      0 --:--:-- --:--:-- --:--:--    25
Warning: Transient problem: HTTP error Will retry in 1 seconds. 3 retries 
Warning: left.
Throwing away 16 bytes
100  219M  100  219M    0     0  6082k      0  0:00:37  0:00:37 --:--:-- 6324k
Building flutter tool...

There is no verbose log either, after I tried -v. Now flutter doctor is stuck at the same place.

What should I do?

Solution

Solved. Seems a network issue. I'm located in China. Had to use StealthVPN with https://pub.flutter-io.cn as the host URL to finish the building. None of the other local mirrors work, which require no StealthVPN.

It's super weird that I can download the Dart SDK from ALL the mirrors but only one of them can complete the "building".

There really should be a clear warning and detailed progress report. It's hard to imagine "building ..." involving internet access.