[BUGFIX beta] Fix various issues with descriptor trap.

[rwjblue]

• Avoid adding enumerable properties to Array.prototype. Ember.NativeArray is a normal Ember.Mixin that we mix into Array.prototype when prototype extensions are enabled. Mutating a native object prototype like this should not result in enumerable properties being added (or we have significant issues with things like deep equality checks from test frameworks, or things like jQuery.extend(true, [], [])). This is a hack, and we should stop mutating the array prototype by default. 😫

• Cache the descriptor trap. Prior to this change, a new proxy was created for every property access. This resulted in some pretty bizarre semantics. Consider the following:

 bar: Ember.computed(function() { });
});

let obj = Foo.create(); console.log(obj.bar === obj.bar);
// => false

O_o

• Avoid adding enumerable properties to String.prototype.
• Avoid adding enumerable properties to Function.prototype.