+str,htc #16071, #16072 New Stream Tcp and Http API

* StreamTcp and Http extensions now return Flows and Sources that can be materialized later
* Flow can now be completed with another flow to be turned into a runnable flow

akka-stream/src/main/scala/akka/stream/scaladsl/Sink.scala

@@ -42,7 +42,7 @@ object Sink {
    * a [[FlowGraphBuilder]] and returns the `UndefinedSource`.
    */
   def apply[T](graph: PartialFlowGraph)(block: FlowGraphBuilder ⇒ UndefinedSource[T]): Sink[T] =
-    createSinkFromBuilder(new FlowGraphBuilder(graph.graph), block)
+    createSinkFromBuilder(new FlowGraphBuilder(graph), block)
 
   private def createSinkFromBuilder[T](builder: FlowGraphBuilder, block: FlowGraphBuilder ⇒ UndefinedSource[T]): Sink[T] = {
     val in = block(builder)