Recently, the web engine build became reliant on prebuilt dart sdks (see flutter/engine#36336, flutter/engine#36851 and #113759).

That means we always use prebuilts and ignore the --no-prebuilt-dart-sdk flag.

This issue is about adding support for the --no-prebuilt-dart-sdk flag. If the flag is set, we build a dart sdk with the default host toolchain.