Previously, only simple, single-slot transclusion worked on upgraded components.
This commit fixes/adds support for the following:
- Multi-slot transclusion.
- Using fallback content when no transclusion content is provided.
- Destroy unused scope (when using fallback content).
Fixes#13271
Previously, non-bracketed inputs (e.g. `xyz="foo"`) on downgraded components
were initialized using `attrs.$observe()` (which uses `$evalAsync()` under the
hood), while bracketed inputs (e.g. `[xyz]="'foo'"`) were initialized using
`$watch()`. If the downgraded component was created during a `$digest` (e.g. by
an `ng-if` watcher), the non-bracketed inputs were not initialized in time for
the initial call to `ngOnChanges()` and `ngOnInit()`.
This commit fixes it by using `$watch()` to initialize all inputs. `$observe()`
is still used for subsequent updates on non-bracketed inputs, because it is more
performant.
Fixes#16212
