R1:4682e0c104bd

Warn against writing to undeclared properties

Summary:
I make this error quite often: I forget to declare a property I am writing to or I make a typo in it.
PHP implicitly creates a public property which I don't like.

I would much rather see a linter warning me against this than this runtime check but writing it is very difficult:

- We need to explore all parents of the class we are checking.
- It is even possible that children will declare that property but it's OK to treat this as error anyway.
- We…