fix spelling in core libraries

sdk/lib/core/list.dart

 // Copyright (c) 2012, 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.core;
 
 /**
  * An indexable collection of objects with a length.
  *
  * Subclasses of this class implement different kinds of lists.
@@ skipping 311 matching lines @@
    *
    * The [iterable] must not have more elements than what can fit from [index]
    * to [length].
    *
    * If `iterable` is based on this list, its values may change /during/ the
    * `setAll` operation.
    */
   void setAll(int index, Iterable<E> iterable);
 
   /**
-   * Removes the first occurence of [value] from this list.
+   * Removes the first occurrence of [value] from this list.
    *
    * Returns true if [value] was in the list, false otherwise.
    *
    *     List<String> parts = ['head', 'shoulders', 'knees', 'toes'];
    *     parts.remove('head'); // true
    *     parts.join(', ');     // 'shoulders, knees, toes'
    *
    * The method has no effect if [value] was not in the list.
    *
    *     // Note: 'head' has already been removed.
@@ skipping 168 matching lines @@
    * as values. The `Map.keys` [Iterable] iterates the indices of this list
    * in numerical order.
    *
    *     List<String> words = ['fee', 'fi', 'fo', 'fum'];
    *     Map<int, String> map = words.asMap();
    *     map[0] + map[1];   // 'feefi';
    *     map.keys.toList(); // [0, 1, 2, 3]
    */
   Map<int, E> asMap();
 }