Use relative URIs for core library part-of statements.

sdk/lib/io/sync_socket.dart

 // Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-part of dart.io;
+part of "io.dart";
 
 /**
  * A low-level class for communicating synchronously over a TCP socket.
  *
  * Warning: [RawSynchronousSocket] should probably only be used to connect to
  * 'localhost'. The operations below will block the calling thread to wait for
  * a response from the network. The thread can process no other events while
  * waiting for these operations to complete. [RawSynchronousSocket] is not
  * suitable for applications that require high performance or asynchronous I/O
  * such as a server. Instead such applications should use the non-blocking
@@ skipping 84 matching lines @@
   /**
    * The [InternetAddress] used to connect this socket.
    */
   InternetAddress get address;
 
   /**
    * The remote [InternetAddress] connected to by this socket.
    */
   InternetAddress get remoteAddress;
 }