When I use the dart plugin and a "Javascript Debug" Run/Debug Configuration (idea config site) and use the sass transformer (sass package) the transformer is not called.
Running pub serve
executes the transformer correctly.
How do I tell Intellij to use the transformer?
There is nothing to do in IntelliJ. Maybe ensure you don't enable the SASS watcher in IntelliJ as well, they might collide. This might be a bug in the SASS transformer or your pubspec.yaml.