fix(upgrade): avoid memory leak when removing downgraded components

Previously, due to the way the AngularJS and Angular clean-up processes
interfere with each other when removing an AngularJS element that
contains a downgraded Angular component, the data associated with the
host element of the downgraded component was not removed. This data was
kept in an internal AngularJS cache, which prevented the element and
component instance from being garbage-collected, leading to memory
leaks.

This commit fixes this by ensuring the element data is explicitly
removed when cleaning up a downgraded component.

NOTE:
This is essentially the equivalent of angular#26209 but for downgraded (instead
of upgraded) components.

Fixes angular#39911
Closes angular#39921

packages/upgrade/src/common/src/angular1.ts

@@ -126,6 +126,7 @@ export type IAugmentedJQuery = Node[]&{
   data?: (name: string, value?: any) => any;
   text?: () => string;
   inheritedData?: (name: string, value?: any) => any;
+  children?: () => IAugmentedJQuery;
   contents?: () => IAugmentedJQuery;
   parent?: () => IAugmentedJQuery;
   empty?: () => void;

packages/upgrade/src/common/src/downgrade_component_adapter.ts

@@ -8,7 +8,7 @@
 
 import {ApplicationRef, ChangeDetectorRef, ComponentFactory, ComponentRef, EventEmitter, Injector, OnChanges, SimpleChange, SimpleChanges, StaticProvider, Testability, TestabilityRegistry, Type} from '@angular/core';
 
-import {IAttributes, IAugmentedJQuery, ICompileService, INgModelController, IParseService, IScope} from './angular1';
+import {element as angularElement, IAttributes, IAugmentedJQuery, ICompileService, INgModelController, IParseService, IScope} from './angular1';
 import {PropertyBinding} from './component_info';
 import {$SCOPE} from './constants';
 import {getTypeName, hookupNgModel, strictEquals} from './util';
@@ -216,11 +216,37 @@ export class DowngradeComponentAdapter {
     const destroyComponentRef = this.wrapCallback(() => this.componentRef.destroy());
     let destroyed = false;
 
-    this.element.on!('$destroy', () => this.componentScope.$destroy());
+    this.element.on!('$destroy', () => {
+      // The `$destroy` event may have been triggered by the `cleanData()` call in the
+      // `componentScope` `$destroy` handler below. In that case, we don't want to call
+      // `componentScope.$destroy()` again.
+      if (!destroyed) this.componentScope.$destroy();
+    });
     this.componentScope.$on('$destroy', () => {
       if (!destroyed) {
         destroyed = true;
         testabilityRegistry.unregisterApplication(this.componentRef.location.nativeElement);
+
+        // The `componentScope` might be getting destroyed, because an ancestor element is being
+        // removed/destroyed. If that is the case, jqLite/jQuery would normally invoke `cleanData()`
+        // on the removed element and all descendants.
+        // - https://github.com/angular/angular.js/blob/2e72ea13fa98bebf6ed4b5e3c45eaf5f990ed16f/src/jqLite.js#L349-L355
+        // - https://github.com/jquery/jquery/blob/6984d1747623dbc5e87fd6c261a5b6b1628c107c/src/manipulation.js#L182
+        //
+        // Here, however, `destroyComponentRef()` will remove the element from the DOM and therefore
+        // it will no longer be a descendant of the removed element when `cleanData()` will be
+        // called. This would result in a memory leak, because the element's data and event handlers
+        // (and all objects directly or indirectly referenced by them) would be retained.
+        //
+        // To ensure the element is always properly cleaned up, we manually call `cleanData()` on
+        // this element and its descendants before destroying the `ComponentRef`.
+        //
+        // NOTE:
+        // `cleanData()` also will invoke the AngularJS `$destroy` event on the element:
+        // - https://github.com/angular/angular.js/blob/2e72ea13fa98bebf6ed4b5e3c45eaf5f990ed16f/src/Angular.js#L1932-L1945
+        angularElement.cleanData(this.element);
+        angularElement.cleanData(this.element.children!());
+
         destroyComponentRef();
       }
     });