Referencing the build directory in index.ts could be problematic

[heruan]

I'm trying V15 and I see now there is a fronted/index.ts file to provide where you instantiate a Flow object passing the path ../target/frontend/generated-flow-imports. For the same reasons discussed in vaadin/flow#6749, referencing the target directory in versioned content can be problematic as it can be different in different build environments.