Add support for cloning server-sockets.

sdk/lib/io/socket.dart

Index: sdk/lib/io/socket.dart
diff --git a/sdk/lib/io/socket.dart b/sdk/lib/io/socket.dart
index b0ace925da5328b64820a84f8cc77ae5b874168d..337029c20f954afbf35c49055639162955c21031 100644
--- a/sdk/lib/io/socket.dart
+++ b/sdk/lib/io/socket.dart
@@ -244,6 +244,36 @@ abstract class RawServerSocket implements Stream<RawSocket> {
    * is fully closed and is no longer bound.
    */
   Future<RawServerSocket> close();
+
+  /**
+   * Get the [RawServerSocketReference].
+   *
+   * WARNING: This feature is *highly experimental* and currently only works on
+   * Linux. The API is most likely going to change in the near future.
+   *
+   * The returned [RawServerSocketReference] can be used to create other
+   * [RawServerSocket]s listening on the same port,
+   * using [RawServerSocketReference.create].
+   * Incoming connections on the port will be distributed fairly between the
+   * active server sockets.
+   * The [RawServerSocketReference] can be distributed to other isolates through
+   * a [RawSendPort].
+   */
+  RawServerSocketReference get reference;
+}
+
+
+/**
+ * A [RawServerSocketReference].
+ *
+ * WARNING: This class is used with [RawServerSocket.reference] which is highly
+ * experimental.
+ */
+abstract class RawServerSocketReference {
+  /**
+   * Create a new [RawServerSocket], from this reference.
+   */
+  Future<RawServerSocket> create();
 }
 
 
@@ -305,8 +335,39 @@ abstract class ServerSocket implements Stream<Socket> {
    * is fully closed and is no longer bound.
    */
   Future<ServerSocket> close();
+
+  /**
+   * Get the [ServerSocketReference].
+   *
+   * WARNING: This feature is *highly experimental* and currently only works on
+   * Linux. The API is most likely going to change in the near future.
+   *
+   * The returned [ServerSocketReference] can be used to create other
+   * [ServerSocket]s listening on the same port,
+   * using [ServerSocketReference.create].
+   * Incoming connections on the port will be distributed fairly between the
+   * active server sockets.
+   * The [ServerSocketReference] can be distributed to other isolates through a
+   * [SendPort].
+   */
+  ServerSocketReference get reference;
+}
+
+
+/**
+ * A [ServerSocketReference].
+ *
+ * WARNING: This class is used with [ServerSocket.reference] which is highly
+ * experimental.
+ */
+abstract class ServerSocketReference {
+  /**
+   * Create a new [ServerSocket], from this reference.
+   */
+  Future<ServerSocket> create();
 }
 
+
 /**
  * The [SocketDirection] is used as a parameter to [Socket.close] and
  * [RawSocket.close] to close a socket in the specified direction(s).