Remove deprecated Strings class.

sdk/lib/_internal/dartdoc/test/markdown_test.dart

 // Copyright (c) 2011, 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.
 
 /// Unit tests for markdown.
 library markdownTests;
 
 // TODO(rnystrom): Use "package:" URL (#4968).
 import '../lib/markdown.dart';
 
@@ skipping 807 matching lines @@
   if (lines.length <= 1) return text;
 
   for (var j = 0; j < lines.length; j++) {
     if (lines[j].length > 8) {
       lines[j] = lines[j].substring(8, lines[j].length);
     } else {
       lines[j] = '';
     }
   }
 
-  return Strings.join(lines, '\n');
+  return lines.join('\n');
 }
 
 validate(String description, String markdown, String html,
          {bool verbose: false}) {
   test(description, () {
     markdown = cleanUpLiteral(markdown);
     html = cleanUpLiteral(html);
 
     var result = markdownToHtml(markdown);
     var passed = compareOutput(html, result);
@@ skipping 35 matching lines @@
 
     // If one string runs out of non-ignored strings, the other must too.
     if (i == a.length) return j == b.length;
     if (j == b.length) return i == a.length;
 
     if (a[i] != b[j]) return false;
     i++;
     j++;
   }
 }