Waiting for another flutter command to release the startup lock
Solution 1:
In my case, following command in Terminal helped (as suggested by Günter):
killall -9 dart
On Windows (as suggested by upupming):
taskkill /F /IM dart.exe
Solution 2:
Remove this file:
<YOUR FLUTTER FOLDER>/bin/cache/lockfile
This releases the occupied lock and makes you able to run other commands.