Move experimental (#2680)

Removes the remaining samples in `experimental`. They're now in the
flutter/demos repository.

*List which issues are fixed by this PR. For larger changes, raising an
issue first helps
reduce redundant work.*

#2520 

## Pre-launch Checklist

- [x] I read the [Flutter Style Guide] _recently_, and have followed its
advice.
- [x] I signed the [CLA].
- [x] I read the [Contributors Guide].
- [x] I have added sample code updates to the [changelog].
- [x] I updated/added relevant documentation (doc comments with `///`).

experimental/federated_plugin/README.md

@@ -1,20 +1,3 @@
-# federated_plugin
+# Sample moved
 
-A Flutter plugin sample that shows how to implement a federated plugin to retrieve current battery level on different platforms.
-
-This sample is currently being built. Not all platforms and functionality are in place.
-
-## Goals for this sample
-
-* Show how to develop a federated plugin which supports Android, iOS, web & desktop.
-* Demonstrate how to use Platform Channels to communicate with different platforms including Web and Desktop.
-
-## Questions/issues
-
-If you have a general question about Flutter, the best places to go are:
-
-* [The FlutterDev Google Group](https://groups.google.com/forum/#!forum/flutter-dev)
-* [The Flutter Gitter channel](https://gitter.im/flutter/flutter)
-* [StackOverflow](https://stackoverflow.com/questions/tagged/flutter)
-
-If you run into an issue with the sample itself, please file an issue [here](https://github.com/flutter/samples/issues).
+The `federated_plugin` sample has been moved to the [flutter/demos][https://github.com/flutter/demos] repository, and is no longer maintained.