Remove StreamSink(replaced by EventSink) and make IOSink extend EventSink.

tests/standalone/io/echo_server_stream_test.dart

Index: tests/standalone/io/echo_server_stream_test.dart
diff --git a/tests/standalone/io/echo_server_stream_test.dart b/tests/standalone/io/echo_server_stream_test.dart
index 2a56e829c0323ac4a0bfb311d18fd6809d5880e7..dfd264d5b2712eb269ff5249409542e46ca31592 100644
--- a/tests/standalone/io/echo_server_stream_test.dart
+++ b/tests/standalone/io/echo_server_stream_test.dart
@@ -64,7 +64,7 @@ class EchoServerGame {
       _socket.listen(onData,
                      onError: errorHandler,
                      onDone: onClosed);
-      _socket.writeBytes(_buffer);
+      _socket.add(_buffer);
       _socket.close();
       data = new List<int>(MSGSIZE);
     }
@@ -121,7 +121,7 @@ class EchoServer extends TestingServer {
           Expect.equals(EchoServerGame.FIRSTCHAR + i, buffer[i]);
         }
         if (offset == MSGSIZE) {
-          connection.writeBytes(buffer);
+          connection.add(buffer);
           connection.close();
         }
       }