SI-4012 Mixin and specialization work well

The bug was fixed along with SI-7638 in 504b5f3.

Author: VladUreche

test/files/pos/SI-4012-a.scala

@@ -0,0 +1,7 @@
+trait C1[+A] {
+  def head: A = sys.error("")
+}
+trait C2[@specialized +A] extends C1[A] {
+  override def head: A = super.head
+}
+class C3 extends C2[Char] 

test/files/pos/SI-4012-b.scala

@@ -0,0 +1,15 @@
+trait Super[@specialized(Int) A] {
+  def superb = 0
+}
+
+object Sub extends Super[Int] {
+  // it is expected that super[Super].superb crashes, since 
+  // specialization does parent class rewiring, and the super
+  // of Sub becomes Super$mcII$sp and not Super. But I consider
+  // this normal behavior -- if you want, I can modify duplicatiors
+  // to make this work, but I consider it's best to keep this
+  // let the user know Super is not the superclass anymore.
+  // super[Super].superb - Vlad
+  super.superb        // okay    
+  override def superb: Int = super.superb // okay
+}