In SERVER-30470, we discovered a bug where field ordering was not preserved when a specific series of oplog entries are applied. Although that bug was resolved after SERVER-50300, we should audit our tests to see if we have field ordering test coverage, and if not, we should create a new jscore test that covers this. The specific ordering of entries in SERVER-30470 that caused an issue was:

Starting with a document { _id: 1, a: 0 };

Apply: o: { $unset: { a: null } }
==> { _id: 1 };
Apply: o: { $set: { 'a.c': 1, b: null } }
==> { _id: 1, a: { c: 1 }, b: null };

// Applying the updates again:
Apply: o: { $unset: { a: null } }
==> { _id: 1, b: null };
Apply: o: { $set: { 'a.c': 1, b: null } }
==> { _id: 1, b: null, a: { c: 1 } };

This came out of a discussion for mongosync regarding field ordering test coverage