Fix broken test

tests/standalone/io/file_test.dart

Index: tests/standalone/io/file_test.dart
diff --git a/tests/standalone/io/file_test.dart b/tests/standalone/io/file_test.dart
index e3b314560436aca454d695c27304e9c4a29ec51b..a94185c37752725ea13411f9aa97d2b7020f4b5c 100644
--- a/tests/standalone/io/file_test.dart
+++ b/tests/standalone/io/file_test.dart
@@ -11,11 +11,19 @@ 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 {