Classes used only in generic positions not recorded in javac analysis

[stuhood]

When a class is used only in a generic position by a consumer, erasure causes it to be "mostly" removed from the classfile. This means that zinc's javac analysis will not recognize the dependency between two classes.

A synthetic example:

public List<Object> emptyList() {
  List<Foo> emptyFooList = new List<Foo>();
  return emptyFooList;
}

But in some positions, the erased type is recorded as a Signature, and could be extracted.