unittest: fix library name for prints matcher

lib/src/prints_matcher.dart

 // Copyright (c) 2014, 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.
 
-library matcher.prints_matcher;
+library unittest.prints_matcher;
 
 import 'dart:async';
 
 import 'package:matcher/matcher.dart' hide completes, wrapAsync, expect;
 
 import '../unittest.dart';
 
 /// Matches a [Function] that prints text that matches [matcher].
 ///
 /// [matcher] may be a String or a [Matcher].
@@ skipping 46 matching lines @@
         .describeMismatch(actual, new StringDescription(), matchState, verbose)
         .toString();
 
     if (innerMismatch.isNotEmpty) {
       description.add('\n   Which: ').add(innerMismatch.toString());
     }
 
     return description;
   }
 }