Fix asset httpServerLocation on Windows.

Summary:
Functions from the path module return paths with the native file system path separator. generateAssetModule is using path.join and path.dirname to generate httpServerLocation.  That means that on Windows systems, the generated URL path has backslashes rather than forward slashes which breaks the app's ability to retrieve image assets using require('./image.png').  This change fixes this by checking path.sep and replacing backslashes with slashes on systems that require it.
Closes #4416

Reviewed By: svcscm

Differential Revision: D2740529

Pulled By: mkonicek

fb-gh-sync-id: edae0f6762c7dc1db7af078209e38a2feab1e0a1

packager/react-packager/src/Bundler/index.js

@@ -364,6 +364,12 @@ class Bundler {
 
   generateAssetModule(bundle, module, platform = null) {
     const relPath = getPathRelativeToRoot(this._projectRoots, module.path);
+    var assetUrlPath = path.join('/assets', path.dirname(relPath));
+
+    // On Windows, change backslashes to slashes to get proper URL path from file path.
+    if (path.sep === '\\') {
+      assetUrlPath = assetUrlPath.replace(/\\/g, '/');
+    }
 
     return Promise.all([
       sizeOf(module.path),
@@ -374,7 +380,7 @@ class Bundler {
       const img = {
         __packager_asset: true,
         fileSystemLocation: path.dirname(module.path),
-        httpServerLocation: path.join('/assets', path.dirname(relPath)),
+        httpServerLocation: assetUrlPath,
         width: dimensions.width / module.resolution,
         height: dimensions.height / module.resolution,
         scales: assetData.scales,