This issue is to track removing a flag that used to be important-ish, but no longer is, and is used across the Flutter codebase.

@bkonyi:

There used to be a slight performance benefit running with --disable-dart-dev when attempting to debug since we used to launch DDS from dartdev, so providing that flag would effectively just tell the VM to skip starting the dartdev isolate and DDS. Now that dart:_vmservice is responsible for starting DDS, this isn't really the case anymore.