fix(router): make setUpLocationChangeListener idempotent

modules/@angular/router/test/router.spec.ts

@@ -6,6 +6,7 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
+import {Location} from '@angular/common';
 import {TestBed} from '@angular/core/testing';
 
 import {ResolveData} from '../src/config';
@@ -32,6 +33,28 @@ describe('Router', () => {
     });
   });
 
+  describe('setUpLocationChangeListener', () => {
+    beforeEach(() => { TestBed.configureTestingModule({imports: [RouterTestingModule]}); });
+
+    it('should be indempotent', () => {
+      const r: Router = TestBed.get(Router);
+      const location: Location = TestBed.get(Location);
+
+      r.setUpLocationChangeListener();
+      const a = (<any>r).locationSubscription;
+      r.setUpLocationChangeListener();
+      const b = (<any>r).locationSubscription;
+
+      expect(a).toBe(b);
+
+      r.dispose();
+      r.setUpLocationChangeListener();
+      const c = (<any>r).locationSubscription;
+
+      expect(c).not.toBe(b);
+    });
+  });
+
   describe('PreActivation', () => {
     const serializer = new DefaultUrlSerializer();
     const inj = {get: (token: any) => () => `${token}_value`};