That hello world takes 17259 lines isn't really a valid point, dart seems to be aimed at building desktop-like applications experiences that happen to be delivered through a web browser. So the valid question is:
What do these 17259 lines enable the developer to do that are impossible or difficult to do with plain old web technologies of HTML, CSS and JavaScript? What's the use case these 17000 enable?
If their aim was to permit building desktoplike applications, they already failed, because the difficulty in building those has almost nothing to do with javascript. The big problem is the DOM api, which is too lowlevel to program large apps in. Javascript's limitations are almost inconsequential next to the deficiency of the DOM api.
Google invents too many programming languages, and too few frameworks / platforms. It's a typical computer scientist response to a software engineering problem. We don't need more languages, we need better api's.
What do these 17259 lines enable the developer to do that are impossible or difficult to do with plain old web technologies of HTML, CSS and JavaScript? What's the use case these 17000 enable?