Remove these extra lines.

Add a "See also:" section here, linking to some other related animated widgets (e.g. AnimatedAlign, AnimatedContainer).

I have added it

There's also a video in some of the other related widgets, like

/// Here's an illustration of what this can look like, using a [curve] of
/// [Curves.fastOutSlowIn].
/// {@animation 250 266 https://flutter.github.io/assets-for-api-docs/assets/widgets/animated_align.mp4}

I wasn't sure how to add those though

Those are added in the flutter/assets-for-api-docs repo, like the one I linked to for AnimatedContainer.

It's not required, but it would be really nice if there were a matching example, like all of the other Animated... widgets have. See animated_container.0.dart for an example.

If you had one, it would be linked here in the docs like so:

/// {@tool dartpad}
/// The following example transitions an [AnimatedFractionallySizedBox]
/// between two states. It adjusts the [heightFactor], [widthFactor], and
/// [alignment] properties when tapped.
///
/// ** See code in examples/api/lib/widgets/implicit_animations/animated_fractionally_sized_box.0.dart **
/// {@end-tool}

Upon adding this example, I got the following error from one of the tests.

The following file does not have the right license header for dart files:
  /b/s/w/ir/x/w/flutter sdk/examples/api/lib/widgets/implicit_animations/animated_fractionally_sized_box.0.dart
The expected license header is:
// Copyright 2014 The Flutter Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
...followed by a blank line.

I added it in so that it passes the test, but since im not a Flutter Author , I'm not sure how this makes sense. Or is that something that the CLA addresses?

You are a Flutter Author if you contribute code to Flutter. If you're wondering about copyright in the CLA, please take a look at the CLA under "Grant of Copyright License".

In any case, all source code needs to have this copyright header.