language tour doesn't explain imports, exports, and part files

[skybrian]

https://www.dartlang.org/docs/dart-up-and-running/ch02.html

Searching on "export" I see that it's a keyword, but not how it works. Similarly for "import".

(More specifically, I was looking for how we explain the format of URL's in import statements.)

[Sfshaza]

This issue was moved to dart-lang/site-www#38