Fix named-amd exports.

Ember does not export named-amd yet. We are shimming the `ember` module to have a single `default` export (this is exactly what EAK and ember-cli expect).

This means that using the following:

```javascript
import {get} from 'ember';
```

Which transpiles to:

```javascript
var get = __dependency1__.get;
```

Will not actually work since `__dependency1__` will only have a single property (`default`) and not a `get` property.

-----

This changes the imports to match reality. At some point, Ember will actually provide real ES6 (and possibly AMD) builds so I kept the variables that we were importing previously stashed at the top of each module (so that when we can do `import {get} from 'ember';` it is as simple as changing these same lines back).
It works similarly to this (it is actually `import { get } from 'ember-metal/property_get';`) in the Ember repo, because we are consuming native ES6 modules (before they are transpiled).

As mentioned in the description of the PR Ember does not generate a native modules build. As such, attempting to import this way will not work.  The globals output is essentially unchanged, as it translates the above line into `var get = Ember.get; var computed = Ember.computed;`.