Fix an error and a warning in unittest.

pkg/unittest/lib/src/interfaces.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 matcher;
 
 // To decouple the reporting of errors, and allow for extensibility of
 // matchers, we make use of some interfaces.
 
 /**
  * The ErrorFormatter type is used for functions that
  * can be used to build up error reports upon [expect] failures.
  * There is one built-in implementation ([defaultErrorFormatter])
  * which is used by the default failure handler. If the failure handler
  * is replaced it may be desirable to replace the [stringDescription]
  * error formatter with another.
  */
 typedef String ErrorFormatter(actual, Matcher matcher, String reason,
     Map matchState, bool verbose);
 
 /**
  * Matchers build up their error messages by appending to
  * Description objects. This interface is implemented by
  * StringDescription. This interface is unlikely to need
  * other implementations, but could be useful to replace in
  * some cases - e.g. language conversion.
  */
 abstract class Description {
+  int get length;
+
   /** Change the value of the description. */
   Description replace(String text);
 
   /** This is used to add arbitrary text to the description. */
   Description add(String text);
 
   /** This is used to add a meaningful description of a value. */
   Description addDescriptionOf(value);
 
   /**
@@ skipping 53 matching lines @@
    * the [reason] (argument from [expect]), some additonal [matchState]
    * generated by the [matcher], and a verbose flag which controls in
    * some cases how much [matchState] information is used. It will use
    * these to create a detailed error message (typically by calling
    * an [ErrorFormatter]) and then call [fail] with this message.
    */
   void failMatch(actual, Matcher matcher, String reason,
                  Map matchState, bool verbose);
 }