Fix broken test

tests/standalone/io/file_test.dart

 // Copyright (c) 2013, 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 "package:expect/expect.dart";
 import 'dart:async';
 import 'dart:collection';
 import 'dart:io';
 import 'dart:isolate';
 
 class MyListOfOneElement
-    extends Object with ListMixin<int>, UnmodifiableListMixin {
+    extends Object with ListMixin<int> implements List<int> {
   int _value;
   MyListOfOneElement(this._value);
   int get length => 1;
   operator [](int index) => _value;
+  void set length(int index) { throw "Unsupported"; }
+  operator []=(int index, value) {
+    if (index != 0) {
+      throw "Unsupported";
+    } else {
+      _value = value;
+    }
+  }
 }
 
 class FileTest {
   static Directory tempDirectory;
   static int numLiveAsyncTests = 0;
   static ReceivePort port;
 
   static void asyncTestStarted() { ++numLiveAsyncTests; }
   static void asyncTestDone(String name) {
     --numLiveAsyncTests;
@@ skipping 1253 matching lines @@
       testDirectorySync();
       testWriteStringUtf8();
       testWriteStringUtf8Sync();
     });
   }
 }
 
 main() {
   FileTest.testMain();
 }