[react-packager] Wait for haste map before accepting any requests

Summary:
D2319999 introduced a regression where we stopped waiting for the "build haste map" step to finish before we accept any requests. This makes sure that we block on that.

Need to unbreak with this, but will follow up with a test to catch this in the future.

packager/react-packager/src/DependencyResolver/DependencyGraph/index.js

@@ -101,7 +101,7 @@ class DependencyGraph {
       this._fastfs.build()
         .then(() => {
           const hasteActivity = Activity.startEvent('Building Haste Map');
-          this._buildHasteMap().then(() => Activity.endEvent(hasteActivity));
+          return this._buildHasteMap().then(() => Activity.endEvent(hasteActivity));
         }),
       this._buildAssetMap_DEPRECATED(),
     ]);