One of the most important features for Dart 2.0, strong mode, has been available for several releases. Migrate to strong mode now to take advantage of the new sound type system that finds errors at compile time (rather than letting your users discover them). You may even find bugs that have been lurking in your code.
If you are using Flutter, it already uses strong mode, so you have met this requirement.
Strong mode is the primary breaking change for 2.0, but there are likely to be other updates. For more information, see Dart 2.0 Updates.