[BUGFIX beta] Make CollectionView _contextBinding one way to eliminate an unnecessary observer
[BUGFIX beta] Avoid defineProperty calls on _context & controller
[BUGFIX beta] `View#element` should not be observable
[BUGFIX beta] Avoid reading current values while setting during initialization

Credit goes to @krisselden for this one. We’re unnecessarily reading values out of the prototype to conditionally notify, but notification is short circuited during initialization inside of propertyWillChange & propertyDidChange.
[BUGFIX beta] `classNameBindings` and `attributeBindings` are concatenatedProperties that should never be a computed property. Therefore, we don’t need to use `get` here.
[BUGFIX beta] It’s possible for `tagName` to be an empty string and therefore we shouldn’t fallback to `get(view, ‘tagName’)`
[BUGFIX beta] Avoid unnecessary work being done for SimpleBoundView due to it not having `tagName: ‘’` specified
Merge pull request #10016 from ebryn/pile-o-perf

A handful of performance tweaks