[Logs] Name fs step

Summary:
There are two fs steps and it wasn't clear why. This now puts the right label:

```
[9:38:25 PM] <START> Building in-memory fs for JavaScript
[9:38:27 PM] <END>   Building in-memory fs for JavaScript (2030ms)
[9:38:27 PM] <START> Building in-memory fs for Assets
[9:38:27 PM] <END>   Building in-memory fs for Assets (615ms)
```

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

@@ -89,10 +89,15 @@ class DependencyGraph {
     });
     this._crawling.then((files) => Activity.endEvent(crawlActivity));
 
-    this._fastfs = new Fastfs(this._opts.roots, this._opts.fileWatcher, {
-      ignore: this._opts.ignoreFilePath,
-      crawling: this._crawling,
-    });
+    this._fastfs = new Fastfs(
+      'JavaScript',
+      this._opts.roots,
+      this._opts.fileWatcher,
+      {
+        ignore: this._opts.ignoreFilePath,
+        crawling: this._crawling,
+      }
+    );
 
     this._fastfs.on('change', this._processFileChange.bind(this));
 
@@ -544,6 +549,7 @@ class DependencyGraph {
     this._assetMap_DEPRECATED = Object.create(null);
 
     const fastfs = new Fastfs(
+      'Assets',
       this._opts.assetRoots_DEPRECATED,
       this._opts.fileWatcher,
       { ignore: this._opts.ignoreFilePath, crawling: this._crawling }

packager/react-packager/src/DependencyResolver/__tests__/Module-test.js

@@ -40,6 +40,7 @@ describe('Module', () => {
   describe('Async Dependencies', () => {
     function expectAsyncDependenciesToEqual(expected) {
       var fastfs = new Fastfs(
+        'test',
         ['/root'],
         fileWatcher,
         {crawling: Promise.resolve(['/root/index.js']), ignore: []},

packager/react-packager/src/DependencyResolver/fastfs.js

@@ -14,8 +14,9 @@ const stat = Promise.denodeify(fs.stat);
 const hasOwn = Object.prototype.hasOwnProperty;
 
 class Fastfs extends EventEmitter {
-  constructor(roots, fileWatcher, {ignore, crawling}) {
+  constructor(name, roots, fileWatcher, {ignore, crawling}) {
     super();
+    this._name = name;
     this._fileWatcher = fileWatcher;
     this._ignore = ignore;
     this._roots = roots.map(root => new File(root, { isDir: true }));
@@ -29,7 +30,7 @@ class Fastfs extends EventEmitter {
     );
 
     return this._crawling.then(files => {
-      const fastfsActivity = Activity.startEvent('Building in-memory fs');
+      const fastfsActivity = Activity.startEvent('Building in-memory fs for ' + this._name);
       files.forEach(filePath => {
         if (filePath.match(rootsPattern)) {
           const newFile = new File(filePath, { isDir: false });