[RFC#706]: Deprecate window.Ember and window.Em, which is an alias

of window.Ember

RFC: emberjs/rfcs#706

tl;dr: Recommend `import Ember from 'ember';` instead of using the window.Ember
global

packages/ember/index.js

@@ -804,7 +804,35 @@ export default Ember;
 if (IS_NODE) {
   module.exports = Ember;
 } else {
-  context.exports.Ember = context.exports.Em = Ember;
+  Object.defineProperty(context.exports, 'Ember', {
+    configurable: true,
+    writable: true,
+    get() {
+      deprecate(
+        "Using window.Ember has been deprecated. Use `import Ember  from 'ember';` instead ",
+        false,
+        {
+          id: 'ember-source.window-global',
+          until: '4.0.0',
+          url: 'tbd',
+          for: 'ember-source',
+          since: { enabled: '3.26.0-beta.1' },
+        }
+      );
+
+      Object.defineProperty(context.exports, 'Ember', {
+        configurable: true,
+        writable: false,
+        get() {
+          return Ember;
+        },
+      });
+
+      return Ember;
+    },
+  });
+
+  context.exports.Em = context.exports.Ember;
 }
 
 /**