## Inheritance Semantics:
Decorators:
1) list the decorators of the class and its parents in the ancestor first order
2) only use the last decorator of each kind (e.g. @Component / ...)
Constructor parameters:
If a class inherits from a parent class and does not declare
a constructor, it inherits the parent class constructor,
and with it the parameter metadata of that parent class.
Lifecycle hooks:
Follow the normal class inheritance model,
i.e. lifecycle hooks of parent classes will be called
even if the method is not overwritten in the child class.
## Example
E.g. the following is a valid use of inheritance and it will
also inherit all metadata:
```
@Directive({selector: 'someDir'})
class ParentDirective {
constructor(someDep: SomeDep) {}
ngOnInit() {}
}
class ChildDirective extends ParentDirective {}
```
Closes#11606Closes#12892
Every decorator now is made of the following:
- a function that can be used
as a decorator or as a constructor. This function
also can be used for `instanceof` checks.
- a type for this function (callable and newable)
- a type that describes the shape of the data
that the user needs to pass to the decorator
as well as the instance of the metadata
The docs for decorators live at the followig places
so that IDEs can discover them correctly:
- General description of the decorator is placed on the
`...Decorator` interface on the callable function
definition
- Property descriptions are placed on the interface
that describes the metadata produces by the decorator
