Add back serializeDocumentToMarkdown

lib/utils/markdown.dart

@@ -31,6 +31,75 @@ MutableDocument deserializeMarkdownToDocument(String markdown) {
   return MutableDocument(nodes: nodeVisitor.content);
 }
 
+String serializeDocumentToMarkdown(Document doc) {
+  final StringBuffer buffer = StringBuffer();
+
+  bool isFirstLine = true;
+  for (int i = 0; i < doc.nodes.length; ++i) {
+    final node = doc.nodes[i];
+
+    if (!isFirstLine) {
+      // Create a new line to encode the given node.
+      buffer.writeln('');
+    } else {
+      isFirstLine = false;
+    }
+
+    if (node is ImageNode) {
+      buffer.write('![${node.altText}](${node.imageUrl})');
+    } else if (node is HorizontalRuleNode) {
+      buffer.write('---');
+    } else if (node is ListItemNode) {
+      final indent = List.generate(node.indent + 1, (index) => '  ').join('');
+      final symbol = node.type == ListItemType.unordered ? '*' : '1.';
+
+      buffer.write('$indent$symbol ${node.text.toMarkdown()}');
+
+      final nodeBelow = i < doc.nodes.length - 1 ? doc.nodes[i + 1] : null;
+      if (nodeBelow != null && (nodeBelow is! ListItemNode)) {
+        // This list item is the last item in the list. Add an extra
+        // blank line after it.
+        buffer.writeln('');
+      }
+    } else if (node is ParagraphNode) {
+      final Attribution blockType = node.getMetadataValue('blockType');
+
+      if (blockType == header1Attribution) {
+        buffer.write('# ${node.text.toMarkdown()}');
+      } else if (blockType == header2Attribution) {
+        buffer.write('## ${node.text.toMarkdown()}');
+      } else if (blockType == header3Attribution) {
+        buffer.write('### ${node.text.toMarkdown()}');
+      } else if (blockType == header4Attribution) {
+        buffer.write('#### ${node.text.toMarkdown()}');
+      } else if (blockType == header5Attribution) {
+        buffer.write('##### ${node.text.toMarkdown()}');
+      } else if (blockType == header6Attribution) {
+        buffer.write('###### ${node.text.toMarkdown()}');
+      } else if (blockType == blockquoteAttribution) {
+        // TODO: handle multiline
+        buffer.write('> ${node.text.toMarkdown()}');
+      } else if (blockType == codeAttribution) {
+        buffer //
+          ..writeln('```') //
+          ..writeln(node.text.toMarkdown()) //
+          ..write('```');
+      } else {
+        buffer.write(node.text.toMarkdown());
+      }
+
+      // Separates paragraphs with blank lines.
+      // If we are at the last node we don't add a trailing
+      // blank line.
+      if (i != doc.nodes.length - 1) {
+        buffer.writeln();
+      }
+    }
+  }
+
+  return buffer.toString();
+}
+
 /// Converts structured markdown to a list of [DocumentNode]s.
 ///
 /// To use [_MarkdownToDocument], obtain a series of markdown