Support circular annotations

With es6-module-loader, it is possible to use circular imports.
Classes from circular imports are not available during the module
evaluation and thus this code fails (`Inject` is undefined):
```js
Foo.annotations = [new Inject];
```

This commit changes the annotation output to use a getter:
```js
Object.defineProperty(Foo, 'annotations', {
  get: function() {
    return [new Inject];
  }
});
```

NOTE:
If the code tries to read `.annotations` or `.parameters` property
during the module evaluation (root context), it won’t work.
Happy to take a PR yes if there are better ways. I do find it a little painful running this for every source, do we really need to clone each time?

Note we should probably move to loading the es6-module-loader as a dependency of Traceur now anyway - I can provide a PR if you agree.