fix(ngClass): do not deconstruct classes on element removal (#10303)

Prior to this fix [ngClass] would remove all dynamic classes
when destroyed. It's essential that classes are persisted such
that remove-based animations will still be stylistically correct.
This patch fixes this issue.

Closes #10008
Closes #10303

modules/@angular/common/src/directives/ng_class.ts

@@ -6,7 +6,7 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
-import {CollectionChangeRecord, Directive, DoCheck, ElementRef, Input, IterableDiffer, IterableDiffers, KeyValueChangeRecord, KeyValueDiffer, KeyValueDiffers, OnDestroy, Renderer} from '@angular/core';
+import {CollectionChangeRecord, Directive, DoCheck, ElementRef, Input, IterableDiffer, IterableDiffers, KeyValueChangeRecord, KeyValueDiffer, KeyValueDiffers, Renderer} from '@angular/core';
 
 import {StringMapWrapper, isListLikeIterable} from '../facade/collection';
 import {isArray, isPresent, isString} from '../facade/lang';
@@ -75,7 +75,7 @@ import {isArray, isPresent, isString} from '../facade/lang';
  * @stable
  */
 @Directive({selector: '[ngClass]'})
-export class NgClass implements DoCheck, OnDestroy {
+export class NgClass implements DoCheck {
   private _iterableDiffer: IterableDiffer;
   private _keyValueDiffer: KeyValueDiffer;
   private _initialClasses: string[] = [];
@@ -129,8 +129,6 @@ export class NgClass implements DoCheck, OnDestroy {
     }
   }
 
-  ngOnDestroy(): void { this._cleanupClasses(this._rawClass); }
-
   private _cleanupClasses(rawClassVal: string[]|Set<string>|{[key: string]: any}): void {
     this._applyClasses(rawClassVal, true);
     this._applyInitialClasses(false);

modules/@angular/core/test/animation/animation_integration_spec.ts

@@ -874,6 +874,39 @@ function declareTests({useJit}: {useJit: boolean}) {
              })));
     });
 
+    describe('ng directives', () => {
+      describe('[ngClass]', () => {
+        it('should persist ngClass class values when a remove element animation is active',
+           inject(
+               [TestComponentBuilder, AnimationDriver],
+               fakeAsync(
+                   (tcb: TestComponentBuilder, driver: InnerContentTrackingAnimationDriver) => {
+                     makeAnimationCmp(
+                         tcb, `<div [ngClass]="exp2" *ngIf="exp" @trigger></div>`,
+                         [
+                           trigger('trigger', [transition('* => void', [animate(1000)])]),
+                         ],
+                         (fixture: any /** TODO #9100 */) => {
+                           var cmp = fixture.debugElement.componentInstance;
+                           cmp.exp = true;
+                           cmp.exp2 = 'blue';
+                           fixture.detectChanges();
+                           flushMicrotasks();
+
+                           expect(driver.log.length).toEqual(0);
+
+                           cmp.exp = false;
+                           fixture.detectChanges();
+                           flushMicrotasks();
+
+                           var animation = driver.log.pop();
+                           var element = animation['element'];
+                           (<any>expect(element)).toHaveCssClass('blue');
+                         });
+                   })));
+      });
+    });
+
     describe('animation states', () => {
       it('should throw an error when an animation is referenced that isn\'t defined within the component annotation',
          inject(

tools/public_api_guard/common/index.d.ts

@@ -291,14 +291,13 @@ export declare const NG_VALIDATORS: OpaqueToken;
 export declare const NG_VALUE_ACCESSOR: OpaqueToken;
 
 /** @stable */
-export declare class NgClass implements DoCheck, OnDestroy {
+export declare class NgClass implements DoCheck {
     initialClasses: string;
     ngClass: string | string[] | Set<string> | {
         [key: string]: any;
     };
     constructor(_iterableDiffers: IterableDiffers, _keyValueDiffers: KeyValueDiffers, _ngEl: ElementRef, _renderer: Renderer);
     ngDoCheck(): void;
-    ngOnDestroy(): void;
 }
 
 /** @experimental */