Important: Advocates don’t need to be developers or
  even full-time team members. They can be designers,
  stakeholders, or external testers that are provided
  with regular builds.

Stay unique

Conforming to expected behaviors doesn’t mean that your app
needs to use default components or styling.
Many of the most popular multiplatform apps have very distinct
and opinionated UIs including custom buttons, context menus,
and title bars.

The more you can consolidate styling and behavior across platforms,
the easier development and testing will be. 
The trick is to balance creating a unique experience with a
strong identity, while respecting the norms of each platform.

Common idioms and norms to consider

Take a quick look at a few specific norms and idioms
you might want to consider, and how you could approach
them in Flutter.

Scrollbar appearance and behavior