Fix paths of module system and polyfills

Reviewed By: cpojer

Differential Revision: D4993289

fbshipit-source-id: d689ed8921c0d07ce2c1301cef3f5a75ff3a6e15

packager/src/ModuleGraph/types.flow.js

@@ -153,7 +153,7 @@ export type TransformedSourceFile =
 export type LibraryOptions = {|
   dependencies?: Array<string>,
   platform?: string,
-  root: string,
+  rebasePath: string => string,
 |};
 
 export type Base64Content = string;