docs(dart): upgrade to beta.22

[chalin]

Dart docs and samples updated.

Sample pubspec.yaml edits:

• upgraded to beta.22
• SDK 1.19.0 is now the minimal version required.

All test suites pass but two (#2492, #2493) after tweaking a few sample titles (to remove mention of the Angular version):

Suites passed:
  public/docs/_examples/architecture/dart
  public/docs/_examples/attribute-directives/dart
  public/docs/_examples/displaying-data/dart
  public/docs/_examples/forms/dart
  public/docs/_examples/lifecycle-hooks/dart
  public/docs/_examples/quickstart/dart
  public/docs/_examples/server-communication/dart
  public/docs/_examples/template-syntax/dart
  public/docs/_examples/toh-1/dart
  public/docs/_examples/toh-2/dart
  public/docs/_examples/toh-3/dart
  public/docs/_examples/toh-4/dart
  public/docs/_examples/toh-5/dart
  public/docs/_examples/toh-6/dart
  public/docs/_examples/user-input/dart
Suites failed:
  public/docs/_examples/dependency-injection/dart
  public/docs/_examples/pipes/dart