This converts existing ## Sample code samples to {@tool sample}...{@end-tool} form.

Also:
1. Fixed a minor bug in analyze-sample-code.dart
2. Made the snippet tool only insert descriptions if the description is non-empty.
3. Moved the Card diagram to before the code sample.

* Future<void> main

* Future<void>.delayed

* prefer_void_to_Null

* address review comments

* enable lint unnecessary_new

* fix tests

* fix tests

* fix tests

* lint unnecessary_new on samples

* fix tests

It was causing problems rolling Flutter into Fuchsia

Revert longPress/Tap and Gallery changes

And add a test to verify we don't do this again.

* Provide haptic/acoustic feedback for tap & long-press on Android

* review comments

* fixed example code

* review comments

* comment fix