onBack stream not closed

## System info

Issue occurs on: both
Plugin version: 0.3.11
Flutter doctor output:

```
Doctor summary (to see all details, run flutter doctor -v):
[√] Flutter (Channel stable, v1.12.13+hotfix.9, on Microsoft Windows [Version 10.0.18363.900], locale en-NZ)

[√] Android toolchain - develop for Android devices (Android SDK version 30.0.0)
[√] Android Studio (version 4.0)
[√] VS Code, 64-bit edition (version 1.46.0)
[√] Connected device (1 available)

• No issues found!
```

## Steps to Reproduce
1. Navigate to the file base.dart in the plugin lib.
2. scroll down to dispose method.


as you can see also in the image, the _onBack stream controller is created but never closed in the dispose. Not sure if this was on purpose or not.

Open lib/base.dart and inspect the dispose method, then trace the _onBack stream controller created there. Confirm whether disposal should close that controller, and verify that the onBack stream is closed when the plugin is disposed.

Written by the indexing model from the issue text.