On macOS and iOS, the build mode is converted to lowercase in the
Flutter-side script, allowing projects to pass configuration names
directly. This adds the same behavior to the tool_backend.dart script
currently used by Windows and Linux, so that Windows builds can do the
same (since the default configuration names used by VS, as in Xcode,
start with a capital letter).
Moves files generated in windows/flutter/ as part of the build to an ephemeral/ subdirectory, matching the approach used on macOS (and in the future, Windows).
Adds that directory to the generated properties file to minimize hard-coding of paths in the project.
Allows Windows builds to use the same structure and script as Linux
builds now use, calling into tool_backend to manage copying resources to
the project directory and building the bundle.
Also switches from expecting name_update.bat to expecting flutter\exe_filename
to be written during the build, as with the recent changes to the macOS build, to
reduce the amount of boilerplate needed in a windows\ project directory.
