Fixes SI-5201. Adds flatten to TraversableViewLike.

Note: This commit exposes a pretty rich type on flatten in views.  HOWEVER, because we don't
capture the higher kinded type of the underlying collection, it makes returning a more minimal type
pretty dang hard.  I can imagine a very breaking and painful change of capturing the underling
collection as a higher-kinded type as well as the current view type in a *ViewLike.scala.

I hope this kind of issue, along with others, drives a rethink of our view API design.

src/library/scala/collection/TraversableViewLike.scala

@@ -162,6 +162,8 @@ trait TraversableViewLike[+A,
 //     if (b.isInstanceOf[NoBuilder[_]]) newFlatMapped(f).asInstanceOf[That]
 //    else super.flatMap[B, That](f)(bf)
   }
+  override def flatten[B](implicit asTraversable: A => /*<:<!!!*/ GenTraversableOnce[B]) = 
+    newFlatMapped(asTraversable)
   private[this] implicit def asThis(xs: Transformed[A]): This = xs.asInstanceOf[This]
 
   /** Boilerplate method, to override in each subclass

test/files/run/t5201.check

@@ -0,0 +1 @@
+true

test/files/run/t5201.scala

@@ -0,0 +1,5 @@
+object Test extends App {
+  val seq = Seq(Seq(1, 2), Seq(3, 4)).view.flatten
+
+  Console.println(seq.isInstanceOf[collection.SeqView[_,_]])
+}