fix spelling in core libraries

sdk/lib/core/expando.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 [Expando] allows adding new properties to objects.
  *
  * Does not work on numbers, strings, booleans or null.
  *
  * An `Expando` does not hold on to the added property value after an object
- * becomes inacessible.
+ * becomes inaccessible.
  *
  * Since you can always create a new number that is identical to an existing
  * number, it means that an expando property on a number could never be
  * released. To avoid this, expando properties cannot be added to numbers.
  * The same argument applies to strings, booleans and null, which also have
  * literals that evaluate to identical values when they occur more than once.
  *
  * There is no restriction on other classes, even for compile time constant
  * objects. Be careful if adding expando properties to compile time constants,
  * since they will stay alive forever.
@@ skipping 31 matching lines @@
   /**
    * Sets the value of this [Expando]'s property on the given
    * object. Properties can effectively be removed again by setting
    * their value to null.
    *
    * The object must not be a number, a string, a boolean or null.
    */
   external void operator []=(Object object, T value);
 
 }