fix spelling in core libraries

sdk/lib/collection/set.dart

 // Copyright (c) 2014, 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.
 
 /**
  * Base implementations of [Set].
  */
 part of dart.collection;
 
 /**
  * Mixin implementation of [Set].
  *
  * This class provides a base implementation of a `Set` that depends only
  * on the abstract members: [add], [contains], [lookup], [remove],
  * [iterator], [length] and [toSet].
  *
  * Some of the methods assume that `toSet` creates a modifiable set.
  * If using this mixin for an unmodifiable set,
  * where `toSet` should return an unmodifiable set,
  * it's necessary to reimplement
  * [retainAll], [union], [intersection] and [difference].
  *
  * Implementations of `Set` using this mixin should consider also implementing
  * `clear` in constant time. The default implementation works by removing every
  * element.
  */
 abstract class SetMixin<E> implements Set<E> {
   // This class reimplements all of [IterableMixin].
   // If/when Dart mixins get more powerful, we should just create a single
-  // Mixin class from IterableMixin and the new methods of thisclass.
+  // Mixin class from IterableMixin and the new methods of this class.
 
   bool add(E element);
 
   bool contains(Object element);
 
   E lookup(Object element);
 
   bool remove(Object element);
 
   Iterator<E> get iterator;
@@ skipping 261 matching lines @@
 abstract class SetBase<E> extends SetMixin<E> {
   /**
    * Convert a `Set` to a string as `{each, element, as, string}`.
    *
    * Handles circular references where converting one of the elements
    * to a string ends up converting [set] to a string again.
    */
   static String setToString(Set set) =>
       IterableBase.iterableToFullString(set, '{', '}');
 }