[BUGFIX release] Avoid positionalParams changing attrs after create.

[rwjblue]

Fixes #11686.

Adds deprecation for:

import Ember from 'ember';

export default Ember.Component.extend({
  positionalParams: [ 'a', 'b' ]
});

Deprecation text is:

Calling `var Thing = Ember.Component.extend({ positionalParams: [\'a\', \'b\' ]});`
is deprecated in favor of `Thing.reopenClass({ positionalParams: [\'a\', \'b\'] });

---

New non-deprecated way:

import Ember from 'ember';

var Thing = Ember.Component.extend();
Thing.reopenClass({
  positionalParams: [ 'a', 'b' ]
});

---

positionalParams was only introduced in Ember 1.13.0 and is not heavily used, I did a GitHub code search for positionalParams and only 35 results were found.

[rwjblue]

/cc @ef4 @stefanpenner

[ef4]

Reviewed, LGTM. Thanks.

[stefanpenner]

sg

[knownasilya]

What's the point of this? Seems like it's just making it harder to use (it wasn't used because it wasn't documented before).

[mixonic]

@knownasilya this is a very power-user feature, and I believe the tradeoff here is ergonomics for performance. Seems a fair swap to me.

[mmun]

The point is that positional params are a static property of the component class. This change enforces that.

[knownasilya]

Makes sense. The guides will have to be updated.