Add e2e tests for Developer Guide Dart examples

[chalin]

• attribute-directives - docs(guide/attribute-directives): follow-up to #1654 #1765
• component-styles
• hierarchical-dependency-injection
• server-communication - docs(server-communication): heavily refactored #1423
• lifecycle-hooks - docs(guide/lifecycle-hooks): follow-up to #1654 #1768
• pipes - docs(dev guide): pipes - new dart prose; updated code; ts updates #1353
• router (once created)
• structural-directives

There are no testing and configuration chapters yet. Contributes to #1521.

[chalin]

This issue was moved to dart-lang/site-webdev#261