[BUGFIX canary] Trust Ember.set

[ef4]

This change eliminates unnecessary dirtying of proxy streams when the
source object is an Ember.Object. It results in significant re-render
performance improvement in certain scenarios, especially when people are
iterating over lists of Ember.Objects to render components with
expensive lifecycle hooks.

We already have a longstanding convention that people need to use set
to mutate Ember.Objects if they expect any observability, and as long as
people are using set, interior mutability will be captured by other
observers further downstream.

The same is not necessarily true for POJOs. We would like to allow
something conceptually like:

myComponent.set('thing', aPojo);
aPojo.deep.inside.here = 'newValue';
myComponent.notifyPropertyChange('thing');

to work correctly, such that the deep mutation gets noticed. Which is
why this change treats POJOs and Ember.Object's differently.

Credit to @wycats for formulating this approach.

[rwjblue]

This seems good (certainly OK to land on canary for us to work through on the way to 2.2.0-beta.1 in a few weeks).

FWIW, I am not a huge fan of the usage of instanceof (it has bit us many times), and would prefer to see something like isEmberObject (which is what we have done for many other things like this).

[rwjblue]

@wycats - r?

[wycats]

@rwjblue big r+ from me. This is one of the two changes I've been harping on since the last Face to Face.