Remove deprecated Strings class.

sdk/lib/_internal/dartdoc/lib/src/markdown/block_parser.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 markdown;
 
 /// The line contains only whitespace or is empty.
 final _RE_EMPTY = new RegExp(r'^([ \t]*)$');
 
 /// A series of `=` or `-` (on the next line) define setext-style headers.
@@ skipping 216 matching lines @@
     return childLines;
   }
 
   Node parse(BlockParser parser) {
     final childLines = parseChildLines(parser);
 
     // The Markdown tests expect a trailing newline.
     childLines.add('');
 
     // Escape the code.
-    final escaped = classifySource(Strings.join(childLines, '\n'));
+    final escaped = classifySource(childLines.join('\n'));
 
     return new Element.text('pre', escaped);
   }
 }
 
 /// Parses horizontal rules like `---`, `_ _ _`, `*  *  *`, etc.
 class HorizontalRuleSyntax extends BlockSyntax {
   RegExp get pattern => _RE_HR;
 
   Node parse(BlockParser parser) {
@@ skipping 20 matching lines @@
 
   Node parse(BlockParser parser) {
     final childLines = [];
 
     // Eat until we hit a blank line.
     while (!parser.isDone && !parser.matches(_RE_EMPTY)) {
       childLines.add(parser.current);
       parser.advance();
     }
 
-    return new Text(Strings.join(childLines, '\n'));
+    return new Text(childLines.join('\n'));
   }
 }
 
 class ListItem {
   bool forceBlock = false;
   final List<String> lines;
 
   ListItem(this.lines);
 }
 
@@ skipping 161 matching lines @@
 
   Node parse(BlockParser parser) {
     final childLines = [];
 
     // Eat until we hit something that ends a paragraph.
     while (!BlockSyntax.isAtBlockEnd(parser)) {
       childLines.add(parser.current);
       parser.advance();
     }
 
-    final contents = parser.document.parseInline(
-        Strings.join(childLines, '\n'));
+    final contents = parser.document.parseInline(childLines.join('\n'));
     return new Element('p', contents);
   }
 }