Webdev + dart install + Flutter + Windows

After a long investigation here https://github.com/dart-lang/build/issues/4330 :)

webdev as it is today will have problems if run using "dart install" and the Flutter SDK on Windows.

This is because

- "dart install" means an AOT compile, which doesn't automatically know how to find the dart binary
- on Windows the Flutter "bin" directory contains "dart.bat", not "dart.exe"

in addition to this it looks like dart.bat has problems correctly handling stdout/stderr, so possibly the right way to support this case is to find dart.bat in path then go from there to where Flutter keeps dart.exe.

Builders that find the Dart binary and launch it, for example to launch the frontend server, will have a similar problem if the builder is compiled with AOT. But I think webdev does not do this yet :)

Reproduce the webdev installation problem with `dart install` and the Flutter SDK on Windows. Start by tracing how webdev and its builders find and launch the Dart binary, including the frontend-server path, then verify behavior with Flutter's `dart.bat` and `dart.exe`. Done means the AOT-installed CLI and relevant builders can locate and launch Dart correctly on Windows.

Written by the indexing model from the issue text.