fix spelling in core libraries

sdk/lib/core/set.dart

 // Copyright (c) 2011, 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;
 
 /**
  * A collection of objects in which each object can occur only once.
  *
  * That is, for each object of the element type, the object is either considered
  * to be in the set, or to _not_ be in the set.
  *
  * Set implementations may consider some elements indistinguishable. These
  * elements are treated as being the same for any operation on the set.
  *
  * The default [Set] implementation, [LinkedHashSet], considers objects
  * indistinguishable if they are equal with regard to
  * operator [Object.==].
  *
  * Iterating over elements of a set may be either unordered
  * or ordered in some way. Examples:
  *
  * * A [HashSet] is unordered, which means that its iteration order is
- *   uspecified,
+ *   unspecified,
  * * [LinkedHashSet] iterates in the insertion order of its elements, and
  * * a sorted set like [SplayTreeSet] iterates the elements in sorted order.
  *
  * It is generally not allowed to modify the set (add or remove elements) while
  * an operation on the set is being performed, for example during a call to
  * [forEach] or [containsAll]. Nor is it allowed to modify the set while
  * iterating either the set itself or any [Iterable] that is backed by the set,
  * such as the ones returned by methods like [where] and [map].
  */
 abstract class Set<E> extends EfficientLengthIterable<E> {
@@ skipping 160 matching lines @@
   /* Creates a [Set] with the same elements and behavior as this `Set`.
    *
    * The returned set behaves the same as this set
    * with regard to adding and removing elements.
    * It initially contains the same elements.
    * If this set specifies an ordering of the elements,
    * the returned set will have the same order.
    */
   Set<E> toSet();
 }