[Web] Normal (non-widgetbook) build adds widgetbook fonts to FontManifest.json

[eggnstone]

Creating a normal (non-widgetbook) Flutter for Web build adds widgetbook fonts to FontManifest.json. This leads to loading error when loading the app in a browser.

Browser console:

main.dart.js:39661 Failed to load font packages/widgetbook/Nunito at assets/packages/widgetbook/google_fonts/Nunito-Black.ttf
main.dart.js:39661 Verify that assets/packages/widgetbook/google_fonts/Nunito-Black.ttf contains a valid font.
main.dart.js:39661 Failed to load font packages/widgetbook/Nunito at assets/packages/widgetbook/google_fonts/Nunito-BlackItalic.ttf
main.dart.js:39661 Verify that assets/packages/widgetbook/google_fonts/Nunito-BlackItalic.ttf contains a valid font.
...

FontManifest.json:

[
{"family":"MaterialIcons","fonts":[{"asset":"fonts/MaterialIcons-Regular.otf"}]},
{"family":"packages/widgetbook/Nunito","fonts":[{"asset":"packages/widgetbook/google_fonts/Nunito-Black.ttf"},{"asset":"packages/widgetbook/google_fonts/Nunito-BlackItalic.ttf"},{"asset":"packages/widgetbook/google_fonts/Nunito-Bold.ttf"},{"asset":"packages/widgetbook/google_fonts/Nunito-BoldItalic.ttf"},{"asset":"packages/widgetbook/google_fonts/Nunito-ExtraBold.ttf"},{"asset":"packages/widgetbook/google_fonts/Nunito-ExtraBoldItalic.ttf"},{"asset":"packages/widgetbook/google_fonts/Nunito-ExtraLight.ttf"},{"asset":"packages/widgetbook/google_fonts/Nunito-ExtraLightItalic.ttf"},{"asset":"packages/widgetbook/google_fonts/Nunito-Italic.ttf"},{"asset":"packages/widgetbook/google_fonts/Nunito-Light.ttf"},{"asset":"packages/widgetbook/google_fonts/Nunito-LightItalic.ttf"},{"asset":"packages/widgetbook/google_fonts/Nunito-Regular.ttf"},{"asset":"packages/widgetbook/google_fonts/Nunito-SemiBold.ttf"},{"asset":"packages/widgetbook/google_fonts/Nunito-SemiBoldItalic.ttf"}]}
]