docs(toh-6/dart): quickfix to "BAD FILENAME" errors (#1929)

Exclude the new Observables section entirely for now.

public/docs/dart/latest/tutorial/toh-pt6.jade

@@ -89,6 +89,9 @@ block heroes-comp-add
 block review
   //- Not showing animated gif due to differences between TS and Dart implementations.
 
+block observables-section
+  //- TBC
+
 block filetree
   .filetree
     .file angular2_tour_of_heroes

public/docs/ts/latest/tutorial/toh-pt6.jade

@@ -359,6 +359,7 @@ block review
   figure.image-display
     img(src='/resources/images/devguide/toh/toh-http.anim.gif' alt="Heroes List Editting w/ HTTP")
 
+block observables-section
   :marked
     ## Observables
 
@@ -554,7 +555,7 @@ block filetree
   - We extended HeroService to support post, put and delete calls.
   - We updated our components to allow adding, editing and deleting of heroes.
   - We configured an in-memory web API.
-  - We learned how to use Observables.
+  <li if-docs="ts"> We learned how to use Observables.</li>
   
   Below is a summary of the files we changed and added.