stuartmorgan fa8b9a5380
Windows text input fixes (#17768)
Fixes a few issues with Windows text input:
- Filters out ASCII control characters
- Filters out lead surrogates, which aren't valid UTF-16 on their own so will cause assertion failures if sent to Flutter
- Adds a bandaid fix for a crash due to mismatches in indexing in the C++ and Dart text models. (A better fix would be to use UTF-16 and add surrogate pair handling to deletion and forward/back; that will be a later PR since it has a larger scope.)

Fixes https://github.com/flutter/flutter/issues/54879
2020-04-20 14:05:41 -07:00
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