don't call endFrame() unless beginFrame() was called

only use condition_variable on non-emscripten builds

thermion_dart/native/include/Log.hpp

@@ -32,7 +32,7 @@ static void Log(const char *fmt, ...) {
 #endif   
     va_end(args);
 }
-#endif
+#endif // ifdef __EMSCRIPTEN__
 
 #if defined(_WIN32) || defined(_WIN64)
 #define __FILENAME__ (strrchr(__FILE__, '\\') ? strrchr(__FILE__, '\\') + 1 : __FILE__)