[BUGFIX release] Ensure _actions specified to extend works.

The initial deprecation of `_actions` and moving it to `actions` was a
bit too naive. The `deprecateProperty` helper was used, which meant that
when `_actions` was set, it would call `set(this, 'actions', newValue)`.
This meant that passing `_actions` to `Ember.Route.extend` (or
`.reopen`) resulted in the `_actions` completely clobbering `actions`
(and loosing any previously defined actions there).

The fix was to:

* Replace the generic `deprecateProperty` usage with a custom
 `Object.defineProperty` call, that defines a setter with an assertion
 instead of clobbering.
* Implementing a `willMergeMixin` that mutates the `props` at extend
  time (to ensure that `_actions` is moved to `actions` with a
  deprecation).

packages/ember-routing/tests/system/route_test.js

@@ -213,6 +213,39 @@ QUnit.test('can access `actions` hash via `_actions` [DEPRECATED]', function() {
   }, 'Usage of `_actions` is deprecated, use `actions` instead.');
 });
 
+QUnit.test('actions in both `_actions` and `actions` results in an assertion', function() {
+  expectAssertion(function() {
+    EmberRoute.extend({
+      _actions: { },
+      actions: { }
+    }).create();
+  }, 'Specifying `_actions` and `actions` in the same mixin is not supported.');
+});
+
+QUnit.test('actions added via `_actions` can be used [DEPRECATED]', function() {
+  expect(3);
+
+  let route;
+  expectDeprecation(function() {
+    route = EmberRoute.extend({
+      _actions: {
+        bar: function() {
+          ok(true, 'called bar action');
+        }
+      }
+    }, {
+      actions: {
+        foo: function() {
+          ok(true, 'called foo action');
+        }
+      }
+    }).create();
+  }, 'Specifying actions in `_actions` is deprecated, please use `actions` instead.');
+
+  route.send('foo');
+  route.send('bar');
+});
+
 QUnit.module('Ember.Route serialize', {
   setup: setup,
   teardown: teardown

packages/ember-runtime/lib/mixins/action_handler.js

@@ -5,7 +5,6 @@
 import Ember from 'ember-metal/core';
 import { Mixin } from 'ember-metal/mixin';
 import { get } from 'ember-metal/property_get';
-import { deprecateProperty } from 'ember-metal/deprecate_property';
 
 /**
   `Ember.ActionHandler` is available on some familiar classes including
@@ -185,13 +184,44 @@ var ActionHandler = Mixin.create({
                    ') does not have a `send` method', typeof target.send === 'function');
       target.send(...arguments);
     }
+  },
+
+  willMergeMixin(props) {
+    Ember.assert('Specifying `_actions` and `actions` in the same mixin is not supported.', !props.actions || !props._actions);
+
+    if (props._actions) {
+      Ember.deprecate(
+        'Specifying actions in `_actions` is deprecated, please use `actions` instead.',
+        false,
+        { id: 'ember-runtime.action-handler-_actions', until: '3.0.0' }
+      );
+
+      props.actions = props._actions;
+      delete props._actions;
+    }
   }
 });
 
 export default ActionHandler;
 
 export function deprecateUnderscoreActions(factory) {
-  deprecateProperty(factory.prototype, '_actions', 'actions', {
-    id: 'ember-runtime.action-handler-_actions', until: '3.0.0'
+  function deprecate() {
+    Ember.deprecate(
+      `Usage of \`_actions\` is deprecated, use \`actions\` instead.`,
+      false,
+      { id: 'ember-runtime.action-handler-_actions', until: '3.0.0' }
+    );
+  }
+
+  Object.defineProperty(factory.prototype, '_actions', {
+    configurable: true,
+    enumerable: false,
+    set(value) {
+      Ember.assert(`You cannot set \`_actions\` on ${this}, please use \`actions\` instead.`);
+    },
+    get() {
+      deprecate();
+      return get(this, 'actions');
+    }
   });
 }