[REQUIRED] Step 2: Describe your environment

• Xcode version: 10.2.1
• Firebase SDK version: 5.20.2
• Firebase Component: Firestore
• Component version: 5.20.2 (I think)

[REQUIRED] Step 3: Describe the problem

When removing documents from a collection, the document changes for those removed documents contain the wrong value for the newIndex. The code states it should be equal to NSNotFound, however it's returning -1 which gets converted to 4294967295.

It's worth noting that oldIndex returns the correct value of NSNotFound.

Steps to reproduce:

1. Add a document to a collection.
2. Remove the document.
3. View the newIndex of that document withing document changes.