Split dart:builtin into dart:builtin and dart:io.

tests/standalone/src/FileTest.dart

 // Copyright (c) 2011, 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.
 //
 // Dart test program for testing file I/O.
 
+#import("dart:io");
 
 class FileTest {
   static Directory tempDirectory;
   static int numLiveAsyncTests = 0;
 
   static void asyncTestStarted() { ++numLiveAsyncTests; }
   static void asyncTestDone() {
     --numLiveAsyncTests;
     if (numLiveAsyncTests == 0) {
       deleteTempDirectory();
@@ skipping 750 matching lines @@
         testAppend();
         testAppendSync();
         asyncTestDone();
       });
   }
 }
 
 main() {
   FileTest.testMain();
 }