docs(cheatsheet/dart): alert reader of possible inaccuracies (#1814)

Until we can generate the cheat sheet properly, at least alert reader
of possible inaccuracies.

Author: chalin

public/docs/dart/latest/cheatsheet.jade

@@ -1,6 +1,8 @@
 - var base = current.path[4] ? '.' : './guide';
 .banner.grid-fluid
-  p.text-body.c10 This cheat sheet is provisional and may change. Angular 2 is currently in Release Candidate.
+  .alert.is-important
+    :marked
+      **Known issue:** Some cheat sheet entries are currently inaccurate, reflecting TypeScript instead of Dart.
 
 article(class="l-content-small grid-fluid docs-content")
   .cheatsheet