Fix observers in 2.3.0+ (#487)

* example

* Reinstate _setupEmberKVO

addon/-private/external/task-decorators.js

@@ -1,7 +1,7 @@
 import { TaskFactory } from './task-factory';
 
 function taskFromPropertyDescriptor(
-  _target,
+  target,
   key,
   descriptor,
   params = [],
@@ -23,6 +23,7 @@ function taskFromPropertyDescriptor(
   let tasks = new WeakMap();
   let options = params[0] || {};
   let factory = new factoryClass(key, taskFn, options);
+  factory._setupEmberKVO(target);
 
   return {
     get() {

tests/unit/decorators-test.js

@@ -1,5 +1,6 @@
 import { module } from 'qunit';
 import { run } from '@ember/runloop';
+import EmberObject from '@ember/object';
 import { setOwner } from '@ember/application';
 import {
   task,
@@ -9,6 +10,7 @@ import {
   enqueueTask,
 } from 'ember-concurrency';
 import { decoratorTest } from '../helpers/helpers';
+import { settled } from '@ember/test-helpers';
 
 module('Unit | decorators', function () {
   decoratorTest('Basic decorators functionality', function (assert) {
@@ -86,4 +88,39 @@ module('Unit | decorators', function () {
     });
     assert.equal(subject.encapsulated.last.value, 56);
   });
+
+  decoratorTest(
+    '`observes` re-performs the task every time the observed property changes in a coalesced manner',
+    async function (assert) {
+      assert.expect(2);
+
+      let values = [];
+      class Obj extends EmberObject {
+        foo = 0;
+
+        @task({ observes: 'foo' })
+        *observingTask() {
+          values.push(this.foo);
+        }
+      }
+
+      let obj = Obj.create();
+      await settled();
+
+      obj.set('foo', 1);
+      obj.set('foo', 2);
+      obj.set('foo', 3);
+      await settled();
+
+      assert.deepEqual(values, [3]);
+      values = [];
+
+      obj.set('foo', 4);
+      obj.set('foo', 5);
+      obj.set('foo', 6);
+      await settled();
+
+      assert.deepEqual(values, [6]);
+    }
+  );
 });