Spec bug: library imports itself doesn't take prefix: into account

[DartBot]

This issue was originally filed by marti...@probst.io

---

The spec (0.08) says:

"It is a compile-time error if a name N is introduced into the library scope of a library A, and either:
N is declared by A, OR
Another import introduces N into the scope of A.
(Note:) This implies that it is a compile-time error for a library to import itself, as the names of its members will be duplicated."

I think this is no longer true given the ability to prefix imports.

[anders-sandholm]

Added Area-Language, Triaged labels.

[gbracha]

It all holds, except that one can import oneself under a prefix. That should be clarified.

---

Set owner to @gbracha.
Added Accepted label.

[anders-sandholm]

Added this to the M1 milestone.

[gbracha]

No longer an issue once import scope is separate from library scope.

---

Added Done label.