pkg/docgen: fixes for unittest deprecations

pkg/docgen/test/generate_json_test.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 docgen.generate_json_test;
 
 import 'dart:io';
 
 import 'package:path/path.dart' as p;
 import 'package:scheduled_test/descriptor.dart' as d;
 import 'package:scheduled_test/scheduled_test.dart';
-import 'package:unittest/matcher.dart';
 
 import 'util.dart';
 import '../lib/docgen.dart' as dg;
 
 void main() {
 
   setUp(() {
     scheduleTempDir();
   });
 
@@ skipping 13 matching lines @@
         d.matcherFile('test_lib-foo.B.json', isJsonMap),
         d.matcherFile('test_lib-foo.json', isJsonMap),
         d.matcherFile('test_lib.A.json', isJsonMap),
         d.matcherFile('test_lib.B.json', isJsonMap),
         d.matcherFile('test_lib.C.json', isJsonMap),
         d.matcherFile('test_lib.json', isJsonMap),
     ]).validate();
 
   });
 }