Changes in dart installation script.

[lrozo]

1. If the user is behind a proxy/firewall, this line may throw an error:
   Cloning into 'dart'... fatal: unable to connect to github.com: github.com: Name or service not known
   Better try cloning via https whenever possible. This also applies to other installation scripts using the same git-clone as this.

2. Typing make -j4 dartpy throws the error:
   make: *** No rule to make target 'dartpy'. Stop.
   I guess it is first needed to move to the python folder and then run the line above.