Fix documentation bugs 7517 and 7518 in serialization.

pkg/serialization/lib/src/reader_writer.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 serialization;
 
 /**
  * This writes out the state of the objects to an external format. It holds
  * all of the intermediate state needed. The primary API for it is the
  * [write] method.
@@ skipping 37 matching lines @@
    * The actual representation of the state is determined by the rule. Lists
    * and Maps are common, but it is arbitrary.
    */
   final List<List> states = new List<List>();
 
   /** Return the list of rules we use. */
   List<SerializationRule> get rules => serialization._rules;
 
   /**
    * Creates a new [Writer] that uses the rules from its parent
-   * [Serialization]. Serializations are do not keep any state
+   * [Serialization]. Serializations do not keep any state
    * related to a particular read/write, so the same one can be used
    * for multiple different Readers/Writers.
    */
   Writer(this.serialization) {
     trace = new Trace(this);
     selfDescribing = serialization.selfDescribing;
   }
 
   /**
    * This is the main API for a [Writer]. It writes the objects and returns
@@ skipping 572 matching lines @@
  * for an example. It knows how to return its object and how to filter.
  */
 class DesignatedRuleForObject {
   Function rulePredicate;
   final target;
 
   DesignatedRuleForObject(this.target, this.rulePredicate);
 
   possibleRules(List rules) => rules.filter(rulePredicate);
 }