In Flutter's integration/driver tests, it is not possible to programmatically type marked text into an iOS text field. Because of this, I couldn't write a regression test for flutter/engine#6989.

The crash happened when marked text was cleared from a Cupertino text field. I was not able to reproduce the crash using our driver tests because of this despite trying various hacks in the engine code. I stopped short of trying something ugly like installing a Japanese keyboard and tapping on the screen at the right location to hit the right onscreen key. My failed attempts are located in #24861.

The docs on UITextInput contain more info on marked text for anyone that's unfamiliar with phonetic input methods like the Japanese Kana keyboard.

CC @HansMuller