Haxe 4 rc3 problems

How do you replace Utf8 with UnicodeString? I’d like to get rid of those annoying warnings.

I have upgraded everything (Haxe, Lime and Openfl) to the dev versions, but now I get

Error: Could not process asset libraries (swflite)

and if I rebuild the tools as explained in this topic I get all the UnicodeString warnings, plus

HaxeToolkit\haxe\lib\format/3,4,2/format/amf3/Tools.hx:73: characters 14-20 : Cannot use abstract as value

error.