Block unsafe use of arrow functions

[schmod]

Arrow functions cannot be safely used in providers or services, because these types are intended to be invoked with the new operator, and expect to have their own value of this.

We should discourage (or outright disallow) people from doing this, because the breakages that it causes could lead to subtle and unpredictable bugs.

[schmod]

Counterpoint: This could be beyond the scope of what this project is meant to provide.

[mgol]

Yeah, I think such things are more in scope of a linter.