Visual Studio 2017 brings a lot of new functionality to the table; as you would expect from a major new release, but Microsoft appear to give with one hand and take away with the other.
One key change I've discovered is that this new language service does not support reference directives, or at least it doesn't appear to work today. By that I mean using something like
Whether this is a bug, or it has been left out of Visual Studio 2017 in favour of you using a module system or TypeScript definitions; I don't know. But I hope we can get it back in a future update.
For now, the great new Go To All feature helps quickly find and check the correct method signatures when I need to call an external method.