- updates tests
- heavy prose revisions
- uses HttpClient (with angular-in-memory-web-api)
- test HeroService using `HttpClientTestingModule`
- scrub away most By.CSS
- fake async observable with `asyncData()`
- extensive Twain work
- different take on retryWhen
- remove app barrels (& systemjs.extras) which troubled plunker/systemjs
- add dummy export const to hero.ts (plunkr/systemjs fails w/o it)
- shrink and re-organize TOC
- add marble testing package and tests
- demonstrate the "no beforeEach()" test coding style
- add section on Http service testing
- prepare for stackblitz
- confirm works in plunker except excluded marble test
- add tests for avoidFile class feature of CodeExampleComponent
PR Close#20697
