Fix HMR on Windows

Summary:Tested HMR on Windows and found 2 small issues related to paths that made it not work. Now it works nicely :)

**Test plan (required)**
Tested HMR in UIExplorer on Windows.
Closes https://github.com/facebook/react-native/pull/6678

Differential Revision: D3138379

fb-gh-sync-id: f27cd2fa21f95954685c8c6916d820f41bc187be
fbshipit-source-id: f27cd2fa21f95954685c8c6916d820f41bc187be

babel-preset/configs/hmr.js

@@ -18,6 +18,12 @@ module.exports = function(options, filename) {
   var transform = filename
       ? './' + path.relative(path.dirname(filename), transformPath) // packager can't handle absolute paths
       : hmrTransform;
+
+  // Fix the module path to use '/' on Windows.
+  if (path.sep === '\\') {
+    transform = transform.replace(/\\/g, '/');
+  }
+
   return {
     plugins: resolvePlugins([
       [

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

@@ -189,23 +189,24 @@ class Bundler {
     );
   }
 
-  _hmrURL(prefix, platform, extensionOverride, path) {
+  _hmrURL(prefix, platform, extensionOverride, filePath) {
-    const matchingRoot = this._projectRoots.find(root => path.startsWith(root));
+    const matchingRoot = this._projectRoots.find(root => filePath.startsWith(root));
 
     if (!matchingRoot) {
-      throw new Error('No matching project root for ', path);
+      throw new Error('No matching project root for ', filePath);
     }
 
-    const extensionStart = path.lastIndexOf('.');
+    // Replaces '\' with '/' for Windows paths.
-    let resource = path.substring(
+    if (path.sep === '\\') {
+      filePath = filePath.replace(/\\/g, '/');
+    }
+
+    const extensionStart = filePath.lastIndexOf('.');
+    let resource = filePath.substring(
       matchingRoot.length,
       extensionStart !== -1 ? extensionStart : undefined,
     );
 
-    const extension = extensionStart !== -1
-      ? path.substring(extensionStart + 1)
-      : null;
-
     return (
       prefix + resource +
       '.' + extensionOverride + '?' +