Revert "Change some platform libraries to use `part of URI`."

sdk/lib/io/process.dart

 // Copyright (c) 2014, 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 dart.io;
 
 // TODO(ager): The only reason for this class is that we
 // cannot patch a top-level at this point.
 class _ProcessUtils {
   external static void _exit(int status);
   external static void _setExitCode(int status);
   external static int _getExitCode();
   external static void _sleep(int millis);
   external static int _pid(Process process);
   external static Stream<ProcessSignal> _watchSignal(ProcessSignal signal);
@@ skipping 577 matching lines @@
   final int errorCode;
 
   const ProcessException(this.executable, this.arguments,
       [this.message = "", this.errorCode = 0]);
   String toString() {
     var msg = (message == null) ? 'OS error code: $errorCode' : message;
     var args = arguments.join(' ');
     return "ProcessException: $msg\n  Command: $executable $args";
   }
 }