REFACTOR: support booting discourse with DISCOURSE_NO_CONSTANTS

This change is discussed here: https://meta.discourse.org/t/deprecating-es6-compatibility-layer/35821

Prior to this change we were not booting correctly with DISCOURSE_NO_CONSTANTS

test/javascripts/lib/bbcode-test.js.es6

@@ -1,4 +1,5 @@
 import Quote from 'discourse/lib/quote';
+import Post from 'discourse/models/post';
 
 module("Discourse.BBCode");
 
@@ -78,7 +79,7 @@ test("size tags", function() {
 
 test("quotes", function() {
 
-  var post = Discourse.Post.create({
+  var post = Post.create({
     cooked: "<p><b>lorem</b> ipsum</p>",
     username: "eviltrout",
     post_number: 1,