fix(collections): clean up UniqueSelectionDispatcher listeners on destroy (#9673)

* Clears out all of the registered listeners in the `UniqueSelectionDispatcher` when the module is unloaded to avoid potential memory leaks due to references inside the listeners.
* Cleans up the unit test setup for the dispatcher to align it with the rest of the project.

src/cdk/collections/unique-selection-dispatcher.spec.ts

@@ -2,31 +2,36 @@ import {UniqueSelectionDispatcher} from './unique-selection-dispatcher';
 
 
 describe('Unique selection dispatcher', () => {
+  let dispatcher: UniqueSelectionDispatcher;
 
-  describe('register', () => {
-    it('once unregistered the listener must not be called on notify', (done) => {
-      let dispatcher: UniqueSelectionDispatcher = new UniqueSelectionDispatcher();
-      let called = false;
+  beforeEach(() => dispatcher = new UniqueSelectionDispatcher());
 
-      // Register first listener
-      dispatcher.listen(() => {
-        called = true;
-      });
+  it('should notify registered listeners', () => {
+    const spy = jasmine.createSpy('listen handler');
 
-      // Register a listener
-      let deregisterFn = dispatcher.listen(() => {
-        done.fail('Should not be called');
-      });
+    dispatcher.listen(spy);
+    dispatcher.notify('id', 'name');
 
-      // Unregister
-      deregisterFn();
+    expect(spy).toHaveBeenCalledWith('id', 'name');
+  });
 
-      // Call registered listeners
-      dispatcher.notify('testId', 'testName');
+  it('should not notify unregistered listeners', () => {
+    const spy = jasmine.createSpy('listen handler');
+    const unregister = dispatcher.listen(spy);
 
-      expect(called).toBeTruthy('Registered listener must be called.');
+    unregister();
+    dispatcher.notify('id', 'name');
 
-      done();
-    });
+    expect(spy).not.toHaveBeenCalled();
+  });
+
+  it('should remove all listeners when destroyed', () => {
+    const spy = jasmine.createSpy('listen handler');
+    dispatcher.listen(spy);
+
+    dispatcher.ngOnDestroy();
+    dispatcher.notify('id', 'name');
+
+    expect(spy).not.toHaveBeenCalled();
   });
 });

src/cdk/collections/unique-selection-dispatcher.ts

@@ -6,7 +6,7 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
-import {Injectable, Optional, SkipSelf} from '@angular/core';
+import {Injectable, Optional, SkipSelf, OnDestroy} from '@angular/core';
 
 
 // Users of the Dispatcher never need to see this type, but TypeScript requires it to be exported.
@@ -22,7 +22,7 @@ export type UniqueSelectionDispatcherListener = (id: string, name: string) => vo
  * less error-prone if they are simply passed through when the events occur.
  */
 @Injectable()
-export class UniqueSelectionDispatcher {
+export class UniqueSelectionDispatcher implements OnDestroy {
   private _listeners: UniqueSelectionDispatcherListener[] = [];
 
   /**
@@ -48,6 +48,10 @@ export class UniqueSelectionDispatcher {
       });
     };
   }
+
+  ngOnDestroy() {
+    this._listeners = [];
+  }
 }
 
 /** @docs-private */