Import of symbolic identifiers does not work

Example:

```
object P {
  def !#@ : Nothing = ???
}

object Test {
  import P.!#@
  def f = !#@
}
```

gives: 

```
importSymbolic.scala:10: error: not found: !#@

  def f = !#@
          ^
one error found
```

The problem is that import selectors are untyped trees, and therefore do not undergo decoding of symbolic characters. The problem will be fixed once we move decoding into a late phase.
