[BUGFIX release] ArrayProxy should not be a computed property

Fixes emberjs#12475.

This commit restores the contract of ArrayProxy content being a simple
property instead of a computed property. This prevents accidental
clobberings such as in ember-data:

https://github.com/emberjs/data/blob/v2.3.3/addon/-private/system/record-arrays/record-array.js#L44

This commit also removes a private hook `arrangedContentArrayWillChange`.
In the interest of keeping this bugfix commit minimal, we will continue
removing the will* hooks on canary.

packages/ember-runtime/lib/system/array_proxy.js

@@ -74,23 +74,7 @@ var ArrayProxy = EmberObject.extend(MutableArray, {
     @type Ember.Array
     @private
   */
-  content: computed({
-    get() {
-      return this._content;
-    },
-    set(k, v) {
-      if (this._didInitArrayProxy) {
-        var oldContent = this._content;
-        var len = oldContent ? get(oldContent, 'length') : 0;
-        this.arrangedContentArrayWillChange(this, 0, len, undefined);
-        this.arrangedContentWillChange(this);
-      }
-      this._content = v;
-      return v;
-    }
-  }),
-
-
+  content: null,
 
   /**
    The array that the proxy pretends to be. In the default `ArrayProxy`
@@ -179,12 +163,16 @@ var ArrayProxy = EmberObject.extend(MutableArray, {
     @method _contentDidChange
   */
   _contentDidChange: observer('content', function() {
-    var content = get(this, 'content');
-    this._teardownContent(this._prevContent);
+    let prevContent = this._prevContent;
+    let content = get(this, 'content');
 
-    assert('Can\'t set ArrayProxy\'s content to itself', content !== this);
+    if (content !== prevContent) {
+      this._teardownContent(prevContent);
 
-    this._setupContent();
+      assert('Can\'t set ArrayProxy\'s content to itself', content !== this);
+
+      this._setupContent();
+    }
   }),
 
   _setupContent() {
@@ -369,7 +357,6 @@ var ArrayProxy = EmberObject.extend(MutableArray, {
   },
 
   init() {
-    this._didInitArrayProxy = true;
     this._super(...arguments);
     this._setupContent();
     this._setupArrangedContent();

packages/ember-runtime/tests/system/array_proxy/content_change_test.js

@@ -36,28 +36,6 @@ QUnit.test('should update length for null content when there is a computed prope
   equal(proxy.get('length'), 0, 'length updates');
 });
 
-QUnit.test('The `arrangedContentWillChange` method is invoked before `content` is changed.', function() {
-  var callCount = 0;
-  var expectedLength;
-
-  var proxy = ArrayProxy.extend({
-    arrangedContentWillChange() {
-      equal(this.get('arrangedContent.length'), expectedLength, 'hook should be invoked before array has changed');
-      callCount++;
-    }
-  }).create({ content: emberA([1, 2, 3]) });
-
-  proxy.pushObject(4);
-  equal(callCount, 0, 'pushing content onto the array doesn\'t trigger it');
-
-  proxy.get('content').pushObject(5);
-  equal(callCount, 0, 'pushing content onto the content array doesn\'t trigger it');
-
-  expectedLength = 5;
-  proxy.set('content', emberA(['a', 'b']));
-  equal(callCount, 1, 'replacing the content array triggers the hook');
-});
-
 QUnit.test('The `arrangedContentDidChange` method is invoked after `content` is changed.', function() {
   var callCount = 0;
   var expectedLength;