If the Flutter SDK is updated we need to invalidate the widget preview
scaffold project to, at the very least, rebuild the precompiled
application. Similarly, if the root project's `pubspec.yaml` is updated,
the scaffold project's pubspec should be regenerated.
This change adds a `preview_manifest.json` to the scaffold project which
contains information related to:
- The manifest schema version
- The Dart SDK version the project was generated with
- The last known hash of the root project's pubspec.yaml
This information is used to determine whether or not the scaffold
project needs to be regenerated or the scaffold pubspec needs to be
updated to reflect changes to the root project's pubspec since the
previewer was last run.
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