I've tried using both master and dev branch, and when runningflutter run -d winuwp -v I get:

[ +27 ms] CMake Error at runner_uwp/CMakeLists.txt:115 (add_executable):
[ +2 ms] Cannot find source file:
[ ] C:/Users/...app/build/winuwp/app.so
[ ] Tried extensions .c .C .c++ .cc .cpp .cxx .cu .mpp .m .M .mm .ixx .cppm .h
[ ] .hh .h++ .hm .hpp .hxx .in .txx .f .F .for .f77 .f90 .f95 .f03 .hip .ispc
[ +15 ms] CMake Error at runner_uwp/CMakeLists.txt:115 (add_executable):
[ ] No SOURCES given to target: app
[ +2 ms] CMake Generate step failed. Build files cannot be regenerated correctly.
[ +11 ms] Building Windows UWP application... (completed in 4.2s)
[ +1 ms] "flutter winuwp" took 4,880ms.
[ +5 ms] Unable to generate build files