[BUGFIX] Setting ArrayProxy#content in willDestroy resets length.

Prior to this change, `ArrayProxy.prototype.length` was marked as dirty
(and therefore recomputed) when `content` was set due to
`arrangedContent` being an alias of `content`, and `ArrayProxy`
implementing `PROPERTY_DID_CHANGE` to trap `arrangedContent` changes.

Unfortunately, `notifyPropertyChange` avoids notifying _some_ things
when the object is destroying. This results in the preexisting
`PROPERTY_DID_CHANGE` trap for `arrangedContent` is no longer called,
and therefore `length` is never marked as dirty.

This fixes the condition, by implementing a `content` trap in
`PROPERTY_DID_CHANGE` that marks length as dirty. The next time that
`length` is accessed it will do the normal work to recalculate.

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

@@ -210,9 +210,7 @@ export default class ArrayProxy extends EmberObject {
     if (content) {
       replace(content, value, removedCount, added);
 
-      this._lengthDirty = true;
-      this._objectsDirtyIndex = 0;
-      this._objects = null;
+      this._invalidate();
     }
   }
 
@@ -225,11 +223,12 @@ export default class ArrayProxy extends EmberObject {
       this._removeArrangedContentArrayObsever();
       this.arrayContentWillChange(0, oldLength, newLength);
 
-      this._objectsDirtyIndex = 0;
-      this._lengthDirty = true;
+      this._invalidate();
 
       this.arrayContentDidChange(0, oldLength, newLength);
       this._addArrangedContentArrayObsever();
+    } else if (key === 'content') {
+      this._invalidate();
     }
   }
 
@@ -277,6 +276,11 @@ export default class ArrayProxy extends EmberObject {
 
     this.arrayContentDidChange(idx, removedCnt, addedCnt);
   }
+
+  _invalidate() {
+    this._objectsDirtyIndex = 0;
+    this._lengthDirty = true;
+  }
 }
 
 ArrayProxy.reopen(MutableArray, {

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

@@ -78,6 +78,24 @@ moduleFor(
       assert.deepEqual(obj.content, null, 'content was updated');
     }
 
+    '@test accessing length after content set to null in willDestroy'(assert) {
+      let obj = ArrayProxy.extend({
+        willDestroy() {
+          this.set('content', null);
+          this._super(...arguments);
+        },
+      }).create({
+        content: ['foo', 'bar'],
+      });
+
+      assert.equal(obj.length, 2, 'precond');
+
+      this.runTask(() => obj.destroy());
+
+      assert.equal(obj.length, 0, 'length is 0 without content');
+      assert.deepEqual(obj.content, null, 'content was updated');
+    }
+
     '@test setting length to 0'(assert) {
       let obj = ArrayProxy.create({ content: ['foo', 'bar'] });