[BUGFIX beta] Object.create(null) in Ember.inspect

This commit resolves an issue where objects passed to Ember.inspect that
contained an `Object.create(null)` object as a property would cause an
exception to be thrown.

I hit this in my app because we were `Ember.inspect`ing objects (a model
in this case) that had their container property set. The container’s
registry as of 1.8.0 is (you guessed it) an object with a null prototype.
[BUGFIX beta] Extracts computed property set into a separate function

This is done to work around V8 (it cannot optimize functions with try
blocks).

Some comparisons from a real app:

`defineProperty`

Before:
![defineproperty
deopt](https://cloud.githubusercontent.com/assets/1131196/4164553/ad42a504-34f6-11e4-92cc-fdf17e88cc22.png)

After:
![defineproperty
cp-set](https://cloud.githubusercontent.com/assets/1131196/4164559/bf012a86-34f6-11e4-9bba-e5b2dc378946.png)

`watchKey`

Before:
![watch - key
deopt](https://cloud.githubusercontent.com/assets/1131196/4164570/e518cc88-34f6-11e4-9d66-97901a75161e.png)

After:
![watch-key -
cp-set](https://cloud.githubusercontent.com/assets/1131196/4164573/ec00d28e-34f6-11e4-8798-222b1ee985bf.png)
Merge pull request #5577 from emberjs/inspect-null-prototype

[BUGFIX beta] Object.create(null) in Ember.inspect
Merge pull request #5547 from twokul/cp-set

[BUGFIX beta] Extracts computed property set into a separate function
Prevent setting aliases on self
[BUGFIX release] make GUID_KEY = intern(GUID_KEY) actually work on ES3.

Ember.keys() filters out __ leading underscores so it returns undefined for GUID_KEY. This change not only works in ES3 browsers but is faster across all browsers I benchmarked.
Merge pull request #5612 from emberjs/fix-intern

[BUGFIX release] make GUID_KEY = intern(GUID_KEY) actually work on ES3.
Include keyName into the assertion error
Merge pull request #5608 from gtsop/master

Assert setting alias on self
[BUGFIX beta] ember-routing: Always inherit model from parent
Allow development only feature flags.

When a flag with `development-only` is found in `features.json` it will
be enabled (flags stripped) for `ember.js`, and disabled (flags
stripped) for `ember.prod.js`.
Fix argument order for handleMandatorySetter.
Use separate FEATURES for dev vs prod builds.
Use `ember-tests.prod.js` when testing prod builds.
Merge pull request #5532 from rwjblue/allow-production-only-feature-flags

Allow development only feature flags.
Merge pull request #5610 from ef4/always-inherit

Fix ember-routing-inherits-parent-model for nestable routes.
Test optional features in production builds.
[FEATURE ember-metal-injected-properties] Ensure tests work in prod builds.
Merge pull request #5618 from rwjblue/run-feature-tests-in-prod

Ensure optional features are tested against production builds.
[BUGFIX release] Cleanup ES3 Object.keys