added WARN_WHEN_USING_OLD_API for fetch as well

packages/mongo/doc_fetcher.js

@@ -1,4 +1,5 @@
 var Fiber = Npm.require('fibers');
+var warnUsingOldApi = require("./mongo_driver").warnUsingOldApi;
 
 export class DocFetcher {
   constructor(mongoConnection) {
@@ -19,9 +20,14 @@ export class DocFetcher {
   fetch(collectionName, id, op, callback) {
     const self = this;
 
+    
     check(collectionName, String);
     check(op, Object);
 
+    // [FIBERS]
+    // TODO: Remove this when 3.0 is released.
+    warnUsingOldApi("fetch");
+
     // If there's already an in-progress fetch for this cache key, yield until
     // it's done and return whatever it returns.
     if (self._callbacksForOp.has(op)) {