Switch from interfaces to abstract classes in dart:io.

runtime/bin/socket_stream.dart

Index: runtime/bin/socket_stream.dart
diff --git a/runtime/bin/socket_stream.dart b/runtime/bin/socket_stream.dart
index 22ab37eef4bd1201d9a5fb1e9ba97aa38b84a84a..668cfe0abbb5dba8fc1dee384f4afe4b369fecc8 100644
--- a/runtime/bin/socket_stream.dart
+++ b/runtime/bin/socket_stream.dart
@@ -6,21 +6,20 @@
  * [SocketInputStream] makes it possible to stream over data received
  * from a [Socket].
  */
-interface SocketInputStream extends InputStream default _SocketInputStream {
+abstract class SocketInputStream implements InputStream {
   /**
    * Create a [SocketInputStream] for streaming from a [Socket].
    */
-  SocketInputStream(Socket socket);
-
+  factory SocketInputStream(Socket socket) => new _SocketInputStream(socket);
 }
 
 /**
  * [SocketOutputStream] makes it possible to stream data to a
  * [Socket].
  */
-interface SocketOutputStream extends OutputStream default _SocketOutputStream {
+abstract class SocketOutputStream implements OutputStream {
   /**
    * Create a [SocketOutputStream] for streaming to a [Socket].
    */
-  SocketOutputStream(Socket socket);
+  factory SocketOutputStream(Socket socket) => new _SocketOutputStream(socket);
 }