Also, add more dartdoc to widgets.dart.

It's safe to remove the unneeded `void`s from setters since the blocking issues in the
`always_declare_return_types` lint have been fixed (https://github.com/dart-lang/linter/).  We can also safely flip the bit on  `avoid_return_types_on_setters`.

This patch renames StatelessComponent to StatelessWidget and
StatefulComponent to StatefulWidget.

Fixes #2308

These are now part of material.dart.

Now SnackBar is an ephemeral route that uses a Placeholder to put itself into
the Scaffold.

Fixes #673