Crash: Emojis + DiffMatchPatch

[jleandroperez]

Details:

We're observing issues when diff'ing surrogate pairs in Simplenote. Ref. here.

Repro steps involve:

1. Setup an entity with a text field
2. Insert the following emojis: ☺️🖖🏿
3. Insert 😃 in between

As a result, an invalid unicode string will be produced, and Apple's JSON parser will end up crashing.