more changes/fixes to get a proper build of a Dart web_ui project done

workflow should look like this:

* pub (single run)
* dwc (in: src/main/dart/web, out: target/generated-sources/dwc)
* dart2js (in: target/generated-sources/dwc, out: target/dart (?))
* [...]
* deployable web app

preferable only one "package" (lib or app) per Maven project