Fix signature of UnmodifiableListMixin.insert to match List.insert.

sdk/lib/internal/list.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.
 
 part of dart._internal;
 
 /**
  * Mixin that throws on the length changing operations of [List].
  *
  * Intended to mix-in on top of [ListMixin] for fixed-length lists.
@@ skipping 105 matching lines @@
         "Cannot modify an unmodifiable list");
   }
 
   /** This operation is not supported by an unmodifiable list. */
   void add(E value) {
     throw new UnsupportedError(
       "Cannot add to an unmodifiable list");
   }
 
   /** This operation is not supported by an unmodifiable list. */
-  E insert(int index, E value) {
+  void insert(int index, E element) {
     throw new UnsupportedError(
         "Cannot add to an unmodifiable list");
   }
 
   /** This operation is not supported by an unmodifiable list. */
   void insertAll(int at, Iterable<E> iterable) {
     throw new UnsupportedError(
         "Cannot add to an unmodifiable list");
   }
 
@@ skipping 246 matching lines @@
  * or as returned by [makeListFixedLength].
  *
  * The operation is efficient. It doesn't copy the elements, but converts
  * the existing list directly to a fixed length list.
  * That means that it is a destructive conversion.
  * The original list should not be used afterwards.
  *
  * The unmodifiable list type is similar to the one used by const lists.
  */
 external List makeFixedListUnmodifiable(List fixedLengthList);