[CDK] Can't use cdkStepper directly since _stepHeader isn't defined

[kherock]

Bug, feature request, or proposal:

Bug

What is the expected behavior?

I should be able to use the [cdkStepper] selector to create a plain stepper component

What is the current behavior?

The FocusKeyManager instance expects to receive a query list it can observe changes on.

Abbreviated stack trace:

ERROR TypeError: Cannot read property 'changes' of undefined
 at FocusKeyManager.ListKeyManager (list-key-manager.ts:60)
 at new FocusKeyManager (focus-key-manager.ts:22)
 at CdkStepper.ngAfterViewInit (stepper.ts:203)

What are the steps to reproduce?

https://stackblitz.com/edit/angular-material2-issue-68xhyx

Which versions of Angular, Material, OS, TypeScript, browsers are affected?

^5.2.4 and ^6.0.0-beta.4

Is there anything else we should know?

I'm guessing if I want to make my own stepper component I should try to copy MatStepper's implementation and extend CdkStepper. However, I'd much rather compose my own stepper using the CDK components since I haven't had great experience extending components in Angular due to the way decorators work.

The CDK stepper lacks any guide for actual usage beyond its API, so it's inconvenient I have to figure out what variables like _stepHeader are for by reading the source code of a fairly complicated component like the material stepper.

Anyway, to fix this I think providing a very basic [cdkStepHeader] directive with the _stepHeader declaration changed to

@ContentChildren(CdkStepHeader) _stepHeader: QueryList<FocusableOption>;

would go a long way.