Implement setPosition on File to seek to a byte position.

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.
 
 
 class FileTest {
   // Test for file read functionality.
   static int testReadStream() {
@@ skipping 256 matching lines @@
     input.openSync();
     input.positionHandler = (position) {
       Expect.equals(0, position);
       List<int> buffer = new List<int>(100);
       input.readListHandler = (bytes_read) {
         input.positionHandler = (position) {
           Expect.equals(12, position);
           input.readListHandler = (bytes_read) {
             input.positionHandler = (position) {
               Expect.equals(18, position);
-              input.close();
+              input.setPositionHandler = () {
+                input.positionHandler = (position) {
+                  Expect.equals(8, position);
+                  input.close();
+                };
+                input.position();
+              };
+              input.setPosition(8);
             };
           };
           input.readList(buffer, 12, 6);
         };
         input.position();
       };
       input.readList(buffer, 0, 12);
     };
     input.position();
     return 1;
   }
 
   static int testPositionSync() {
     String filename = getFilename("tests/vm/data/fixed_length_file");
     File input = new File(filename);
     input.openSync();
     Expect.equals(0, input.positionSync());
     List<int> buffer = new List<int>(100);
     input.readListSync(buffer, 0, 12);
     Expect.equals(12, input.positionSync());
     input.readListSync(buffer, 12, 6);
     Expect.equals(18, input.positionSync());
+    input.setPositionSync(8);
+    Expect.equals(8, input.positionSync());
     input.closeSync();
     return 1;
   }
 
   // Tests exception handling after file was closed.
   static int testCloseException() {
     bool exceptionCaught = false;
     bool wrongExceptionCaught = false;
     String filename = getFilename("tests/vm/data/fixed_length_file");
     File input = new File(filename + "_out");
@@ skipping 253 matching lines @@
     Expect.equals(1, testCloseException());
     Expect.equals(1, testCloseExceptionStream());
     Expect.equals(1, testBufferOutOfBoundsException());
     Expect.equals(1, testMixedSyncAndAsync());
   }
 }
 
 main() {
   FileTest.testMain();
 }