docs(server-comm): fix path to heroes.json for Dart

public/docs/ts/_cache/guide/server-communication.jade

@@ -648,8 +648,9 @@ a#in-mem-web-api
 :marked
   ## Appendix: Tour of Heroes in-memory server
 
-  If we only cared to retrieve data, we could tell Angular to get the heroes from a `heroes.json` file like this one:
-+makeJson('server-communication/ts/app/heroes.json', null, 'app/heroes.json')(format=".")
+  If the app only needed to retrieve data, you could get the heroes from a `heroes.json` file:
+- var _heroesJsonPath = (_docsFor == 'dart' ? 'web' : 'app') + '/heroes.json';
++makeJson('server-communication/' + _docsFor + '/' + _heroesJsonPath, null, _heroesJsonPath)(format=".")
 .l-sub-section
   :marked
     We wrap the heroes array in an object with a `data` property for the same reason that a data server does:

public/docs/ts/latest/guide/server-communication.jade

@@ -653,7 +653,8 @@ a#in-mem-web-api
   ## Appendix: Tour of Heroes in-memory server
 
   If the app only needed to retrieve data, you could get the heroes from a `heroes.json` file:
-+makeJson('server-communication/ts/app/heroes.json', null, 'app/heroes.json')(format=".")
+- var _heroesJsonPath = (_docsFor == 'dart' ? 'web' : 'app') + '/heroes.json';
++makeJson('server-communication/' + _docsFor + '/' + _heroesJsonPath, null, _heroesJsonPath)(format=".")
 .l-sub-section
   :marked
     You wrap the heroes array in an object with a `data` property for the same reason that a data server does: