firebase-ios-sdk

Upgrade to Firestore Emulator 1.5.0 (#3171)

This includes fixes for:

  * All the field escaping tests
  * testCanWriteVeryLargeBatches
  * testWritesWithIndirectlyNestedArraysSucceed

All tests now pass. I've left isRunningAgainstEmulator in the source
even though there are now no callers.