When running dart from flutter-sdk/bin/dart, the FLUTTER_ROOT variable is set (the same is true for flutter).

DevTools sometimes needs to call Flutter commands but in the extension we always run the dart executable directly (not through the shell script) so this isn't set. We should set it for all tools we spawn (when we are in Flutter mode, with a Flutter SDK) to make it easier for them to locate the same Flutter version the IDE is using.