New rule: prevent assignment of existing properties on a service (important for tests)

[NullVoxPopuli]

I've seen some code that does:

let service = this.owner.lookup('service:foo');

service.setProperties({ 
  replacement properties and methods
});
// or
service.foo = replacement
// or
Ember.set(service, 'foo', replacement)

we should lint against this, because it can cause sutle bugs.

Additionally, you end up testing your replacements, rather than the service itself.

If someone wants to

• override network stuff
  • use MSW or similar (let the fetch happen)
• override some method that uses an external library
  • couple options:
  • let it happen, assert your full integration of the tools
  • extend the service

    class Replacement extends Foo {
    get foo() {
      assert.step('ran foo');
      // optionally
      return super.foo;
    }
    }
    this.owner.register('service:foo', Replacement);