Fix babel only regexp on windows

Summary:
cli / packager was broken on Windows after 28f1c67ced.

Babel `only` regexp option requires paths to use forward slashes only but `__dirname` will use backslashes on windows. This simply adds a replace to normalize to forward slashes.

cc jeanlauliac cpojer
Closes https://github.com/facebook/react-native/pull/11850

Reviewed By: cpojer

Differential Revision: D4409143

Pulled By: jeanlauliac

fbshipit-source-id: a705236630959f762e53100299ab073ec9a29ee1

setupBabel.js

@@ -26,7 +26,9 @@ const BABEL_ENABLED_PATHS = [
  */
 function buildRegExps(basePath, dirPaths) {
   return dirPaths.map(folderPath =>
-    new RegExp(`^${escapeRegExp(path.resolve(basePath, folderPath))}`)
+    // Babel `only` option works with forward slashes in the RegExp so replace
+    // backslashes for Windows.
+    new RegExp(`^${escapeRegExp(path.resolve(basePath, folderPath).replace(/\\/g, '/'))}`)
   );
 }