[BUGFIX release] Ensure model can be observed by sync observers

This is a compatibility change, otherwise sync observers
will not be able to watch the model property of controllers
at all. Also fixes the naming of the final flag to Observable#addObserver
and Observable#removeObserver

packages/@ember/-internals/runtime/lib/mixins/observable.js

@@ -362,12 +362,12 @@ export default Mixin.create({
     @param {String} key The key to observe
     @param {Object} target The target object to invoke
     @param {String|Function} method The method to invoke
-    @param {Boolean} async Whether the observer is async or not
+    @param {Boolean} sync Whether the observer is sync or not
     @return {Observable}
     @public
   */
-  addObserver(key, target, method, async) {
-    addObserver(this, key, target, method, async);
+  addObserver(key, target, method, sync) {
+    addObserver(this, key, target, method, sync);
     return this;
   },
 
@@ -380,12 +380,12 @@ export default Mixin.create({
     @param {String} key The key to observe
     @param {Object} target The target object to invoke
     @param {String|Function} method The method to invoke
-    @param {Boolean} async Whether the observer is async or not
+    @param {Boolean} sync Whether the observer is async or not
     @return {Observable}
     @public
   */
-  removeObserver(key, target, method, async) {
-    removeObserver(this, key, target, method, async);
+  removeObserver(key, target, method, sync) {
+    removeObserver(this, key, target, method, sync);
     return this;
   },
 

packages/@ember/controller/lib/controller_mixin.js

@@ -1,7 +1,10 @@
-import { Mixin, tracked } from '@ember/-internals/metal';
+import { Mixin, computed } from '@ember/-internals/metal';
 import { ActionHandler } from '@ember/-internals/runtime';
+import { symbol } from '@ember/-internals/utils';
 import { EMBER_METAL_TRACKED_PROPERTIES } from '@ember/canary-features';
 
+const MODEL = symbol('MODEL');
+
 /**
 @module ember
 */
@@ -43,5 +46,13 @@ export default Mixin.create(ActionHandler, {
     @property model
     @public
   */
-  model: EMBER_METAL_TRACKED_PROPERTIES ? tracked() : null,
+  model: computed({
+    get() {
+      return this[MODEL];
+    },
+
+    set(key, value) {
+      return (this[MODEL] = value);
+    },
+  }),
 });

packages/@ember/controller/tests/controller_test.js

@@ -3,7 +3,70 @@ import Service, { inject as injectService } from '@ember/service';
 import { Object as EmberObject } from '@ember/-internals/runtime';
 import { Mixin, get } from '@ember/-internals/metal';
 import { runDestroy, buildOwner } from 'internal-test-helpers';
-import { moduleFor, AbstractTestCase } from 'internal-test-helpers';
+import { moduleFor, ApplicationTestCase, AbstractTestCase, runTask } from 'internal-test-helpers';
+import { action } from '@ember/object';
+
+moduleFor(
+  'Controller model',
+  class extends ApplicationTestCase {
+    async '@test model is tracked'(assert) {
+      this.add(
+        'controller:index',
+        class extends Controller {
+          constructor() {
+            super(...arguments);
+            this.model = 0;
+          }
+
+          get derived() {
+            return this.model + 1;
+          }
+
+          @action
+          update() {
+            this.model++;
+          }
+        }
+      );
+
+      this.addTemplate('index', '<button {{on "click" this.update}}>{{this.derived}}</button>');
+
+      await this.visit('/');
+
+      this.assertText('1');
+
+      runTask(() => this.$('button').click());
+      this.assertText('2');
+    }
+
+    async '@test model can be observed with sync observers'(assert) {
+      let observerDidRun = false;
+
+      this.add(
+        'controller:index',
+        class extends Controller {
+          constructor() {
+            super(...arguments);
+            this.model = 0;
+
+            this.addObserver('model', this, () => (observerDidRun = true), true);
+          }
+
+          @action
+          update() {
+            this.model++;
+          }
+        }
+      );
+
+      this.addTemplate('index', '<button {{on "click" this.update}}>{{this.model}}</button>');
+
+      await this.visit('/');
+      runTask(() => this.$('button').click());
+      assert.equal(observerDidRun, true, 'observer ran');
+    }
+  }
+);
 
 moduleFor(
   'Controller event handling',