How to write/cite a description of a software product in a master's thesis?
If you wish to quote the text verbatim, then you could write -- as you suggest -- "According to the developer's website, 'Dart is an open-source...' [citation]"

Bear in mind however, that you are writing a research thesis. You should be demonstrating your ability to discriminate, evaluate and criticise. In your short example, there are some statements which are open to debate. Are the libraries indeed robust? Is the development paradigm indeed non-dogmatic? Do you, as a learned academic user of the software, fully agree with these statements or can you offer some additional insight?