Make mtime optional on ReadDirItem, change utime to mtime on StatResult

Android and iOS both set  for stat method, so this type declaration must be updated. Making mtime optional on ReadDirItem fixes #378

Author: autoreleasefool

FS.common.js

@@ -33,7 +33,7 @@ type MkdirOptions = {
 
 type ReadDirItem = {
   ctime: ?Date;    // The creation date of the file (iOS only)
-  mtime: Date;   // The last modified date of the file
+  mtime: ?Date;    // The last modified date of the file
   name: string;     // The name of the item
   path: string;     // The absolute path to the item
   size: string;     // Size in bytes
@@ -47,7 +47,7 @@ type StatResult = {
   size: string;     // Size in bytes
   mode: number;     // UNIX file mode
   ctime: number;    // Created date
-  utime: number;    // Updated date
+  mtime: number;    // Last modified date
   isFile: () => boolean;        // Is the file just a file?
   isDirectory: () => boolean;   // Is the file a directory?
 };
@@ -165,7 +165,7 @@ function readDirGeneric(dirpath: string, command: Function) {
   return command(normalizeFilePath(dirpath)).then(files => {
     return files.map(file => ({
       ctime: file.ctime && new Date(file.ctime * 1000) || null,
-      mtime: new Date(file.mtime * 1000),
+      mtime: file.mtime && new Date(file.mtime * 1000) || null,
       name: file.name,
       path: file.path,
       size: file.size,