Remove deprecated Strings class.

samples/markdown/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 markdown_tests;
 
 import '../lib.dart';
 
 // TODO(rnystrom): Better path to unittest.
@@ skipping 760 matching lines @@
 cleanUpLiteral(String text) {
   var lines = text.split('\n');
   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');
 }
 
 /// Does a loose comparison of the two strings of HTML. Ignores differences in
 /// newlines and indentation.
 compareOutput(String a, String b) {
   int i = 0;
   int j = 0;
 
   skipIgnored(String s, int i) {
     // Ignore newlines.
@@ skipping 12 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++;
   }
 }