Add missing newline after Actual value in fail message.

pkg/unittest/lib/src/string_matchers.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;
 
 /**
  * Returns a matcher which matches if the match argument is a string and
  * is equal to [value] when compared case-insensitively.
  */
@@ skipping 170 matching lines @@
 
   bool matches(item, Map matchState) =>
     item is String ? _regexp.hasMatch(item) : false;
 
   Description describe(Description description) =>
       description.add("match '${_regexp.pattern}'");
 }
 
 // String matchers match against a string. We add this intermediate
 // class to give better mismatch error messages than the base Matcher class.
-abstract class _StringMatcher extends BaseMatcher {
+abstract class _StringMatcher extends Matcher {
   const _StringMatcher();
   Description describeMismatch(item, Description mismatchDescription,
                                Map matchState, bool verbose) {
     if (!(item is String)) {
       return mismatchDescription.
           addDescriptionOf(item).
           add(' not a string');
     } else {
       return super.describeMismatch(item, mismatchDescription,
           matchState, verbose);
     }
   }
 }