fix(auth_backend): update logout method to return a promise

Closes angular#583

Author: ryanclark

src/auth/auth.ts

@@ -119,8 +119,8 @@ export class AngularFireAuth extends ReplaySubject<FirebaseAuthState> {
     }
   }
 
-  public logout(): void {
-    this._authBackend.unauth();
+  public logout(): Promise<void> {
+    return this._authBackend.unauth();
   }
 
   public getAuth(): FirebaseAuthState {

src/auth/auth_backend.ts

@@ -11,7 +11,7 @@ export abstract class AuthBackend {
     : Promise<FirebaseAuthState>;
   abstract onAuth(): Observable<FirebaseAuthState>;
   abstract getAuth(): FirebaseAuthState;
-  abstract unauth(): void;
+  abstract unauth(): Promise<void>;
   abstract createUser(credentials: EmailPasswordCredentials): Promise<FirebaseAuthState>;
   abstract getRedirectResult(): Observable<firebase.auth.UserCredential>;
 }

src/auth/firebase_sdk_auth_backend.ts

@@ -67,8 +67,8 @@ export class FirebaseSdkAuthBackend extends AuthBackend {
     return observeOn.call(authState, new ZoneScheduler(Zone.current));
   }
 
-  unauth(): void {
-    Promise.resolve(this._fbAuth.signOut());
+  unauth(): Promise<void> {
+    return <Promise<void>>this._fbAuth.signOut();
   }
 
   authWithCustomToken(token: string): Promise<FirebaseAuthState> {