Revert "Revert "DEV: Import MessageBus from message-bus-client instead of globals""

This reverts commit b10e995d9d.

test/javascripts/acceptance/topic-discovery-test.js

@@ -1,4 +1,6 @@
 import { acceptance } from "helpers/qunit-helpers";
+import MessageBus from "message-bus-client";
+
 acceptance("Topic Discovery", {
   settings: {
     show_pinned_excerpt_desktop: true
@@ -85,7 +87,7 @@ QUnit.test("Live update unread state", async assert => {
   );
 
   // Mimic a messagebus message
-  window.MessageBus.callbacks.filterBy("channel", "/latest").map(c =>
+  MessageBus.callbacks.filterBy("channel", "/latest").map(c =>
     c.func({
       message_type: "read",
       topic_id: 11995,

test/javascripts/test_helper.js

@@ -17,7 +17,7 @@
 
 // Stuff we need to load first
 //= require vendor
-//= require ember-shim
+//= require discourse-shims
 //= require pretty-text-bundle
 //= require markdown-it-bundle
 //= require application
@@ -55,8 +55,10 @@ sinon.config = {
 
 window.inTestEnv = true;
 
+let MessageBus = require("message-bus-client").default;
+
 // Stop the message bus so we don't get ajax calls
-window.MessageBus.stop();
+MessageBus.stop();
 
 // Trick JSHint into allow document.write
 var d = document;
@@ -191,7 +193,7 @@ QUnit.testDone(function() {
     window.Discourse.__container__
   );
 
-  window.MessageBus.unsubscribe("*");
+  MessageBus.unsubscribe("*");
   delete window.server;
   window.Mousetrap.reset();
 });