Default arg exports

The DCE info register was tested locally with the following code:

```
@JSExport
object HelloWorld {
  @JSExport
  def main() = {
    val a = (new A).asInstanceOf[js.Dynamic]
    println(a.foo())
  }
}

class A {
  @JSExport
  def foo(x: String = "the default string") = x
}
```

Before f40e3073e4d0fbaa0e946f1f124693d371ffc8bd, the call to the default argument getter of `A.foo` fails after DCE, since it has been removed. After the commit it works.