Move markdown library.

utils/markdown/html_renderer.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.
-
-String renderToHtml(List<Node> nodes) => new HtmlRenderer().render(nodes);
-
-/// Translates a parsed AST to HTML.
-class HtmlRenderer implements NodeVisitor {
-  static final _BLOCK_TAGS = const RegExp(
-      'blockquote|h1|h2|h3|h4|h5|h6|hr|p|pre');
-
-  StringBuffer buffer;
-
-  HtmlRenderer();
-
-  String render(List<Node> nodes) {
-    buffer = new StringBuffer();
-
-    for (final node in nodes) node.accept(this);
-
-    return buffer.toString();
-  }
-
-  void visitText(Text text) {
-    buffer.add(text.text);
-  }
-
-  bool visitElementBefore(Element element) {
-    // Hackish. Separate block-level elements with newlines.
-    if (!buffer.isEmpty() &&
-        _BLOCK_TAGS.firstMatch(element.tag) != null) {
-      buffer.add('\n');
-    }
-
-    buffer.add('<${element.tag}');
-
-    for (final name in element.attributes.getKeys()) {
-      buffer.add(' $name="${element.attributes[name]}"');
-    }
-
-    if (element.isEmpty) {
-      // Empty element like <hr/>.
-      buffer.add(' />');
-      return false;
-    } else {
-      buffer.add('>');
-      return true;
-    }
-  }
-
-  void visitElementAfter(Element element) {
-    buffer.add('</${element.tag}>');
-  }
-}