Fix package analysis detecting web-incompatible

[GZGavinZhao]

The Dart Package ANAlysis system can't recognize this package as js-compatible because of the file lib/builder.dart that uses dart:io. By moving the file to lib/src/builder.dart and stop exporting it, this should stop PANA from detecting it and hence reporting our package as web-compatible.

[github-actions[bot]]

Visit the preview URL for this PR (updated for commit 6eea1d2):

https://angulardart-gallery--pr61-fix-sass-builder-u0nqace5.web.app

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