export default broken for Class expression with target ES6 and commonjs module

Using typescript@next and the following compiler options:

``` json
"compilerOptions": {
  "target": "ES6",
  "module": "commonjs",
}
```

A class expression being default exported will be incorrectly compiled into a named export instead.

Example:

``` typescript
export default class Foo {}
```

compiles into:

``` javascript
class Foo {
}
exports.Foo = Foo;
```
