add post-1.0 language features to language tour

Staged at:

https://langtour-dot-dart-lang.appspot.com/docs/dart-up-and-running/ch02.html#enums

https://langtour-dot-dart-lang.appspot.com/docs/dart-up-and-running/ch02.html#deferred-loading

https://langtour-dot-dart-lang.appspot.com/docs/dart-up-and-running/ch02.html#asynchrony

R=sethladd@google.com

Review URL: https://chromiumcodereview.appspot.com/757003003