docs(dart): upgrade to beta.20

[chalin]

All currently active E2E tests pass except for lifecycle-hooks which fails because of recent updates to that test file (it is fixed in #2141):

Suites passed:
  public/docs/_examples/architecture/dart
  public/docs/_examples/attribute-directives/dart
  public/docs/_examples/dependency-injection/dart
  public/docs/_examples/displaying-data/dart
  public/docs/_examples/forms/dart
  public/docs/_examples/pipes/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/user-input/dart
Suites failed:
  public/docs/_examples/lifecycle-hooks/dart

cc @kwalrath

[kwalrath]

LGTM