Validate that documentation code is formatted

This updates the `VerifyDiagnosticsTest` to additionally verify that
the code in the diagnostic documentation is correctly formatted.

There is a global flag that can be used to temporarily disable the
verification in order to make it easier to roll a new version of
`dart_style` into the Dart sdk when doing so would involve making too
many changes.

There is also a mechanism added to allow some code snippets to not be
verified. There are times when requiring the code to be formatted isn't
reasonable, either because the invalid code can't be parsed which would
prevent the formatter from running, or because the code would become
confusing to readers. Exceptions should be documented for future
reference. And if the code later conforms to the formatter, the lack
of need for skipping it will be flagged.

Change-Id: I3689940d4479e7b635e290acc8d3857d96ab5200
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/524820
Commit-Queue: Brian Wilkerson <brianwilkerson@google.com>
Reviewed-by: Samuel Rawlins <srawlins@google.com>
Reviewed-by: Paul Berry <paulberry@google.com>