Unmapper Windows compatibility (facebook#3079)

* Switch to unix path separators before normalizing path for Windows compatibility

* Add comment for posterity

* Revert "Add comment for posterity"

This reverts commit 742bace.

* Strictly add comment

packages/react-error-overlay/src/utils/unmapper.js

@@ -56,14 +56,19 @@ async function unmap(
     }
     let { fileName } = frame;
     if (fileName) {
-      fileName = path.normalize(fileName);
+      // The web version of this module only provides POSIX support, so Windows
+      // paths like C:\foo\\baz\..\\bar\ cannot be normalized.
+      // A simple solution to this is to replace all `\` with `/`, then
+      // normalize afterwards.
+      fileName = path.normalize(fileName.replace(/[\\]+/g, '/'));
     }
     if (fileName == null) {
       return frame;
     }
     const fN: string = fileName;
     const source = map
       .getSources()
+      // Prepare path for normalization; see comment above for reasoning.
       .map(s => s.replace(/[\\]+/g, '/'))
       .filter(p => {
         p = path.normalize(p);