Fix Ember global deprecation

A minimal fix for #600. We should probably follow through with the
refactor suggested by @rwjblue in #600 but that can be done later.

This is probably not very embroider friendly, but neither is the
current code. It's probably easier to just rewrite the shims in
terms of ES modules and embroider macros, i.e. what was suggested
in #337.

assets/browser-fetch.js.t

@@ -1,7 +1,14 @@
 (function (originalGlobal) {
-  define('fetch', ['exports'], function(exports) {
+  if (!require.has('ember')) {
+    define('ember', ['exports'], function(exports) {
+      exports['default'] = originalGlobal.Ember;
+    });
+  }
+
+  define('fetch', ['exports', 'ember'], function(exports, Ember) {
     'use strict';
-    var Promise = originalGlobal.Ember.RSVP.Promise;
+    var Ember__default = 'default' in Ember ? Ember['default'] : Ember;
+    var Promise = Ember__default.RSVP.Promise;
     var supportProps = [
       'FormData',
       'FileReader',
@@ -49,8 +56,8 @@
       return result;
     }
 
-    if (originalGlobal.Ember.Test) {
-      originalGlobal.Ember.Test.registerWaiter(function() {
+    if (Ember__default.Test) {
+      Ember__default.Test.registerWaiter(function() {
         return pending === 0;
       });