Install this package:
emerge -a virtual/flutter
This virtual package provides the following dependencies:
<pkgmetadata>
<maintainer type="person">
<email>gentoo@arran4.com</email>
<name>Arran Ubels</name>
</maintainer>
<longdescription>
Allows downstream packages to use either the source-built Flutter SDK
or the binary Flutter SDK (dev-lang/flutter-bin).
</longdescription>
</pkgmetadata>