Add e2e tests for all Dart examples

[chalin]

This issue adds e2e tests for Dart by reusing the TS tests. In some cases it merely consists of enabling the tests. In other cases, when e2e tests fail, it is usually an indication that the Dart source is out of sync with the TS example source.

• quickstart - test(e2e/dart): add support for Dart e2e testing #1369
• tutorial (toh) - Add e2e tests for Tutorial Dart examples #1619
• basics (developer guide) - Add e2e tests for Basics Dart examples #1522
• developer guide - Add e2e tests for Developer Guide Dart examples #1523

[chalin]

Only #1523 remains. Closing this parent issue.