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.

* Improve documentation

* Potential performance win in hit testing

Rather than copying the list every time we access `path`, this just
exposes it as an `Iterable`. People who want to copy it can use
`toList()`.

(This isn't a breaking change since code that expects a `List` is
still going to get one.)

* Enforce the Oxford Comma.

* 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

This adds a simple spacer widget first suggested by Scott Stoll (@ScottS2017) in the abandoned PR #15802

This PR replaces that one, cleans it up, and adds a test.