dartium 38663 (new formula)

Added suport to install the latest stable dartium browser from
https://www.dartlang.org. The browser is used to develop dart
applications and is common installed with the dart sdk provided by
dart.rb Formula
Don't use a `name` variable, just write it inline. Also, this looks like it's binary software so should be either compiled from source or submitted to https://github.com/Homebrew/homebrew-binary, perhaps even `homebrew-cask`.