[FEATURE ember-routing-linkto-target-attribute] Enable by default.
[FEATURE ember-routing-auto-location-uses-replace-state-for-history]

Enable by default.
[FEATURE ember-metal-is-present] Enable by default.
[FEATURE ember-runtime-proxy-mixin] Remove feature (make private).

* Remove feature flags.
* Make `Ember._ProxyMixin` private API for internal use.
Merge pull request #5389 from rwjblue/enable-linkto-target-attribute

[FEATURE ember-routing-linkto-target-attribute] Enable by default.
Merge pull request #5390 from rwjblue/ember-routing-auto-location-uses-replace-state-for-history

[FEATURE ember-routing-auto-location-uses-replace-state-for-history]
Merge pull request #5391 from rwjblue/enable-ember-metal-is-present

[FEATURE ember-metal-is-present] Enable by default.
Merge pull request #5392 from rwjblue/remove-proxy-mixin-feature

[FEATURE ember-runtime-proxy-mixin] Remove feature (make private).
[FEATURE property-brace-expansion-improvement] Enable by default.

Also, updates `Ember.expandProperties` docs.