Fix JS compilation to use the command 'compile js' instead of using
snapshot names to invoke dart2js
The Dart SDK is switching all tools to use AOT snapshots instead of JIT
snapshots and using snapshot names directly causes things to break.
Fixes https://github.com/flutter/flutter/issues/130277
This PR does two things:
1. introduce a hidden `flutter build _preview` command, that will build a debug windows desktop app and copy it into the SDK's binary cache. This command is only intended to be run during packaging.
2. introduce a new device type, called `PreviewDevice`, which relies on the prebuilt desktop debug app from step 1, copies it into the target app's assets build folder, and then hot reloads their dart code into it.
