BREAKING CHANGE:
- Injector was renamed into `ReflectiveInjector`,
as `Injector` is only an abstract class with one method on it
- `Injector.getOptional()` was changed into `Injector.get(token, notFoundValue)`
to make implementing injectors simpler
- `ViewContainerRef.createComponent` now takes an `Injector`
instead of `ResolvedProviders`. If a reflective injector
should be used, create one before calling this method.
(e.g. via `ReflectiveInjector.resolveAndCreate(…)`.
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