[BUGFIX beta] Bring back (with deprecation) Ember.EXTEND_PROTOTYPES.

packages/ember/index.js

@@ -110,6 +110,7 @@ import Resolver from '@ember/application/globals-resolver';
 import ApplicationInstance from '@ember/application/instance';
 import Engine from '@ember/engine';
 import EngineInstance from '@ember/engine/instance';
+import { deprecate } from 'ember-debug';
 
 // ****ember-environment****
 
@@ -131,6 +132,17 @@ Object.defineProperty(Ember, 'lookup', {
   enumerable: false,
 });
 
+Object.defineProperty(Ember, 'EXTEND_PROTOTYPES', {
+  enumerable: false,
+  get() {
+    deprecate(
+      'Accessing Ember.EXTEND_PROTOTYPES is deprecated, please migrate to Ember.ENV.EXTEND_PROTOTYPES'
+    );
+
+    return ENV.EXTEND_PROTOTYPES;
+  },
+});
+
 // ****@ember/application****
 Ember.getOwner = getOwner;
 Ember.setOwner = setOwner;

packages/ember/tests/reexports_test.js

@@ -21,6 +21,16 @@ moduleFor(
     ['@test Ember.String.isHTMLSafe exports correctly'](assert) {
       confirmExport(Ember, assert, 'String.isHTMLSafe', 'ember-glimmer', 'isHTMLSafe');
     }
+
+    ['@test Ember.EXTEND_PROTOTYPES is present (but deprecated)'](assert) {
+      expectDeprecation(() => {
+        assert.strictEqual(
+          Ember.ENV.EXTEND_PROTOTYPES,
+          Ember.EXTEND_PROTOTYPES,
+          'Ember.EXTEND_PROTOTYPES exists'
+        );
+      }, /EXTEND_PROTOTYPES is deprecated/);
+    }
   }
 );