refactor(database): import utility from angularfire2

Author: adriancarriger

src/database.ts

@@ -3,6 +3,7 @@
  */ /** */
 import { Inject, Injectable } from '@angular/core';
 import { AngularFire, FirebaseListObservable, FirebaseObjectObservable } from 'angularfire2';
+import { isNil } from 'angularfire2/utils';
 import { FirebaseListFactoryOpts, FirebaseObjectFactoryOpts } from 'angularfire2/interfaces';
 
 import { AfoListObservable } from './afo-list-observable';
@@ -301,14 +302,6 @@ export class AngularFireOfflineDatabase {
   }
 }
 /**
- * Utility function used to check if an value exists.
- */
-export function isNil(obj: any): boolean {
-  return obj === undefined || obj === null;
-}
-/**
- * The AngularFire2 unwrap function.
- *
  * Adds the properies of `$key`, `$value`, `$exists` as required by AngularFire2
  */
 export function unwrap(key: string, value: any, exists) {