Test cases closes SI-4574.

Looks like I got that irrefutability bug too.

test/files/run/t4574.check

@@ -0,0 +1,2 @@
+I hereby refute null!
+I denounce null as unListLike!

test/files/run/t4574.scala

@@ -0,0 +1,13 @@
+object Test {
+  val xs: List[(Int, Int)] = List((2, 2), null)
+
+  def expectMatchError[T](msg: String)(body: => T) {
+    try { body ; assert(false, "Should not succeed.") }
+    catch { case _: MatchError => println(msg) }
+  }
+
+  def main(args: Array[String]): Unit = {
+    expectMatchError("I hereby refute null!")( for ((x, y) <- xs) yield x )
+    expectMatchError("I denounce null as unListLike!")( (null: Any) match { case List(_*) => true } )
+  }
+}