Waiting for another flutter command to release the startup lock

In my case, the following command in Terminal helped (as suggested by Günter Zöchbauer's comment):

killall -9 dart

On Windows, run the following in a Command Prompt or PowerShell window (as suggested by upupming's comment):

taskkill /F /IM dart.exe

(You can read about taskkill flags in Windows here)

Remove this file:

<YOUR FLUTTER FOLDER>/bin/cache/lockfile

This releases the occupied lock and makes you able to run other commands.