Use pub for execution of build runner

[jmewes]

Solves #178

[jmewes]

I have never use the code generation before, but in the following article I learned that pub is used for the execution of the build runner:

https://medium.com/flutter-community/part-1-code-generation-in-dart-the-basics-3127f4c842cc

And in contrast to the documented commands this actually generates e.g. the server.dart file.

[jmewes]

The tests fail even though only Markdown files were changed. So I assume the problem is in the tests themselves.