Change output of string equality match failures.

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.
 
 /**
@@ skipping 35 matching lines @@
 /**
  * [expect] Matchers must implement the Matcher class.
  * The base Matcher class that implements this interface has
  * a generic implementation of [describeMismatch] so this does
  * not need to be provided unless a more clear description is
  * required. The other two methods ([matches] and [describe])
  * must always be provided as they are highly matcher-specific.
  */
 abstract class Matcher {
   /**
+   * Whether failures should include the actual value in the output.
+   * The reason is passed in so that it can be inspected if necessary
+   * for some more complex cases.
+   */
+  bool includeActual(reason) => true;
+
+  /**
    * This does the matching of the actual vs expected values.
    * [item] is the actual value. [matchState] can be supplied
    * and may be used to add details about the mismatch that are too
    * costly to determine in [describeMismatch].
    */
   bool matches(item, MatchState matchState);
 
   /** This builds a textual description of the matcher. */
   Description describe(Description description);
 
@@ skipping 26 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,
                  MatchState matchState, bool verbose);
 }