Add missing newline after Actual value in fail message.

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 28 matching lines @@
 
   /**
    * This is used to add a description of an [Iterable] [list],
    * with appropriate [start] and [end] markers and inter-element [separator].
    */
   Description addAll(String start, String separator, String end,
                      Iterable list);
 }
 
 /**
- * [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
+ * [expect] Matchers must implement/extend the Matcher class.
+ * The base Matcher class 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 {
+  const Matcher();
+
   /**
    * 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, Map matchState);
 
   /** This builds a textual description of the matcher. */
   Description describe(Description description);
 
   /**
    * This builds a textual description of a specific mismatch. [item]
    * is the value that was tested by [matches]; [matchState] is
    * the [Map] that was passed to and supplemented by [matches]
    * with additional information about the mismact, and [mismatchDescription]
    * is the [Description] that is being built to decribe the mismatch.
    * A few matchers make use of the [verbose] flag to provide detailed
    * information that is not typically included but can be of help in
    * diagnosing failures, such as stack traces.
    */
   Description describeMismatch(item, Description mismatchDescription,
-      Map matchState, bool verbose);
+      Map matchState, bool verbose) => mismatchDescription;
 }
 
 /**
  * Failed matches are reported using a default IFailureHandler.
  * The default implementation simply throws [TestFailure]s;
  * this can be replaced by some other implementation of
  * IFailureHandler by calling configureExpectHandler.
  */
 abstract class FailureHandler {
   /** This handles failures given a textual decription */
   void fail(String reason);
 
   /**
    * This handles failures given the actual [value], the [matcher]
    * 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);
 }