[BUGFIX beta] Warn rather than raise readOnly CPs

[tomdale]

Previously, CPs with setters would raise an exception if they were
marked as readOnly. This change broke Semantic Versioning because
it caused working app code to break.

For example, if you had a computed property that was read-only, but you
happened to include a second value argument out of habit or
convention, this change would cause your previously-functioning app to
break:

computed(function(key, value) {
  // value is unused
}).readOnly()

The above code example will cause an exception to be raised.

In Ember 2.0 we can change this deprecation back to an assertion.

[rwjblue]

The build failed due to bizarre Travis cache issue. I cleared the cache, and restarted the build.

[rwjblue]

👍 on changing this to a deprecation. We will also likely need to add a blurb about this deprecation to the deprecation guide.

[cibernox]

Before merging this, check #10968. I've changed the assertion check to not affect people using the old CP syntax, only people using the new one.

I do think that calling readOnly over a CP with a setter is land for an assertion rather than a deprecation, the problem is that the change was backwards incompatible. Now it should be safe.

[rwjblue]

@cibernox - I agree that #10968 seems preferable.

@tomdale - Can you review and confirm?

[stefanpenner]

Ya I prefer @cibernox update

[rwjblue]

Fixed with #10968.