Change output of string equality match failures.

pkg/unittest/lib/src/interfaces.dart

Index: pkg/unittest/lib/src/interfaces.dart
===================================================================
--- pkg/unittest/lib/src/interfaces.dart	(revision 23039)
+++ pkg/unittest/lib/src/interfaces.dart	(working copy)
@@ -53,6 +53,13 @@
  */
 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