The official Flutter assets and further guidelines on representing the brand
can be found at Representing the Flutter Brand.

General Rules That Govern the Use of the Flutter Trademarks

You are free to use the Flutter trademarks: (i) in connection with your
download and use of the Flutter SDK to build and develop
apps, (ii) in training materials (e.g., video tutorials, online publications,
etc.) that provide instructions or tips regarding how to use the Flutter
SDK to build and develop apps, and (iii) to show your support for
the use of the Flutter SDK by members of the developer community.

These guidelines do not restrict your right to use the “Flutter” name
in connection with descriptions of the Flutter SDK that would be
considered “fair use.”  For example, you may use the “Flutter”
name to make truthful factual statements (e.g., “built with the Flutter
SDK”) or to accurately describe a feature of the Flutter SDK.