Dart is still maturing as an environment, so if you start working in it you should expect the odd issue.
Dart has a command 'Pub Deploy' to create a regular JS web deployable version of your project. It works great for a first release of such a big feature. Unless you are using GIT. 10983: Pub deploy fails if out/ is in .gitignore.
Easy enough to get .gitignore out the way, deploy and then put it back. A fairly painless workaround. Enough of a frustrating GOTCHA to be worth highlighting. Makes me wonder where they are going long term with this tool. I don't think DEPLOY should depend on a particular VCS. Odd.