Use before define from variables to classes.

This code (from Angular):

``` dart
/// Marker for an uninitialized value.
const UNINITIALIZED = const _Uninitialized();
class _Uninitialized { const _Uninitialized(); }
```

is causing a runtime error on generated code as the use is before the define:

``` javascript
  let UNINITIALIZED = new _Uninitialized();
  class _Uninitialized extends core.Object {
    _Uninitialized() {
    }
  }
```
