[Angular Elements]: Adding ComponentNgElementStrategyFactory & ComponentNgElementStrategy to public API

[jKelio]

I'm submitting a...

[ ] Regression (a behavior that used to work and stopped working in a new release)
[ ] Bug report  
[ ] Performance issue
[X] Feature request
[ ] Documentation issue or request
[ ] Support request => Please do not submit support request here, instead see https://github.com/angular/angular/blob/master/CONTRIBUTING.md#question
[ ] Other... Please describe:

Current behavior

On implementing an optional custom strategy provider, when I create a new custom element with an angular component, I am forced to implement a complete new custom strategy provider with an own factory.

Expected behavior

But if I want to hook up a custom behavior to the default ComponentNgElementStrategy, I have to copy&paste the default class. Maybe it would be better, to import the default class that the custom element strategy could inherit from the default one.

Minimal reproduction of the problem with instructions

I have pushed an angular elements example with a custom behavior hooked up into the default copied and pasted ComponentNgElementStrategy on the connect method:
https://github.com/jKelio/angular-element-example

What is the motivation / use case for changing the behavior?

Avoiding too much coping&pasting stuff from the angular framework itself.

Environment

Angular version: 6.1.0

[remackgeek]

This would be useful. While waiting for the zone changes to land, I had to do some hacky things to be able to wrap ComponentNgElementStrategy. See elements-zone-strategy

[totszwai]

We need this to workaround the issue mentioned in #29606

[gkalpak]

We have decided to not make ComponentNgElementStrategy part of the public API. See #37416 (comment) for more info.

If you need to define your own strategy, you can copy ComponentNgElementStrategy (and the corresponding factory).

[angular-automatic-lock-bot]

This issue has been automatically locked due to inactivity.
Please file a new issue if you are encountering a similar or related problem.

Read more about our automatic conversation locking policy.

_{This action has been performed automatically by a bot.}