[BUGFIX beta] Prevent willDestroyElement from being called twice.

The node-managers ensure that `willDestroyElement` (and other hooks)
are called properly (by walking the much more precise `view._renderNode`
tree), having the renderer's `willDestroyElement` also traverse `childViews`
leads to `willDestroyElement` getting called twice in cases other than those
for `ContainerView`.

packages/ember-htmlbars/tests/integration/will-destroy-element-hook-test.js

@@ -0,0 +1,60 @@
+import run from 'ember-metal/run_loop';
+import Component from 'ember-views/views/component';
+import compile from 'ember-template-compiler/system/compile';
+import { runAppend, runDestroy } from "ember-runtime/tests/utils";
+
+import { set } from 'ember-metal/property_set';
+
+var component;
+
+QUnit.module('ember-htmlbars: destroy-element-hook tests', {
+  teardown() {
+    runDestroy(component);
+  }
+});
+
+QUnit.test('willDestroyElement is only called once when a component leaves scope', function(assert) {
+  var done = assert.async();
+  var innerChild, innerChildDestroyed;
+
+  component = Component.create({
+    switch: true,
+
+    layout: compile(`
+     {{~#if switch~}}
+       {{~#view innerChild}}Truthy{{/view~}}
+     {{~/if~}}
+    `),
+
+    innerChild: Component.extend({
+      init() {
+        this._super(...arguments);
+        innerChild = this;
+      },
+
+      willDestroyElement() {
+        if (innerChildDestroyed) {
+          throw new Error('willDestroyElement has already been called!!');
+        } else {
+          innerChildDestroyed = true;
+        }
+      }
+    })
+  });
+
+  runAppend(component);
+
+  assert.equal(component.$().text(), 'Truthy', 'precond - truthy template is displayed');
+  assert.equal(component.get('childViews.length'), 1);
+
+  run.later(function() {
+    set(component, 'switch', false);
+
+    run.later(function() {
+      assert.equal(innerChild.get('isDestroyed'), true, 'the innerChild has been destroyed');
+      assert.equal(component.$().text(), '', 'truthy template is removed');
+
+      done();
+    });
+  });
+});