SI-7200 Test case for fixed type inference error.

Broken in 2.9.2 and 2.10.0, but working in 2.10.1

    --- sandbox/2.10.0.log
    +++ sandbox/2.10.1.log
           def coflatMap[A >: Nothing <: Any, B >: Nothing <: Any](f: Test.Nel[A] => B): Test.Nel[A] => Test.Nel[B] = ((l: Test.Nel[A]) => Test.this.Nel.apply[B](f.apply(l), l.tail match {
             case immutable.this.Nil => immutable.this.Nil
    -        case (hd: A, tl: List[A])scala.collection.immutable.::[A]((h @ _), (t @ _)) => {
    -          val r: Test.Nel[Nothing] = NelFoo.this.coflatMap[A, Nothing](f).apply(Test.this.Nel.apply[A](h, t));
    +        case (hd: A, tl: List[A])scala.collection.immutable.::[?A1]((h @ _), (t @ _)) => {
    +          val r: Test.Nel[B] = NelFoo.this.coflatMap[A, B](f).apply(Test.this.Nel.apply[A](h, t));
               {
    -            <synthetic> val x$1: Nothing = r.head;
    -            r.tail.::[Nothing](x$1)
    +            <synthetic> val x$1: B = r.head;
    +            r.tail.::[B](x$1)
               }
             }
           }))

b74c33eb86 represents the exact moment of progression. Comments
in pos/t7200b.scala, a minimal test that demonstrates the problem
without type constructors or code execution, pinpoint the line of
code responsible for the fix.

Incidentally, I'm currently on a train somewhere between Solothurn
and Biel, and am consequently without the power of scala-bisector.
Undeterred, and inspired by a line I saw in Skyfall last night
("sometimes the olds ways are better"), I pulled of a two-hop
bisection. Take that, O(log N)!

The one remaining worry is the appearance of the type variable
?A1 in the output of -Xprint:typer for run/t7200.scala.