[analyzer] Parse fenced code blocks in doc comments

This simplifies comment reference-parsing in this refactor.
Basically when a comment reference is discovered, we
immediately parse it into a CommentReferenceImpl, instead of
passing around the offset from place to place.

Additionally we store fenced code block data on each Comment,
for highlighting purposes. More data will be parsed in this
code in the future. In particular, doc imports.

Work towards #50702

Change-Id: Idb2dcae3fb54af567b1a7f43896c3226644bf6cc
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/317446
Commit-Queue: Samuel Rawlins <srawlins@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>

pkg/analyzer/lib/src/dart/ast/ast.dart

@@ -3225,6 +3225,10 @@ sealed class CombinatorImpl extends AstNodeImpl implements Combinator {
 ///        '/ **' (CHARACTER | [CommentReference])* '*/'
 ///      | ('///' (CHARACTER - EOL)* EOL)+
 abstract final class Comment implements AstNode {
+  /// The fenced code blocks parsed in this comment.
+  @experimental
+  List<MdFencedCodeBlock> get fencedCodeBlocks;
+
   /// Return `true` if this is a block comment.
   bool get isBlock;
 
@@ -3270,6 +3274,9 @@ final class CommentImpl extends AstNodeImpl implements Comment {
   /// within it.
   final NodeListImpl<CommentReferenceImpl> _references = NodeListImpl._();
 
+  @override
+  final List<MdFencedCodeBlock> fencedCodeBlocks;
+
   /// Initialize a newly created comment. The list of [tokens] must contain at
   /// least one token. The [_type] is the type of the comment. The list of
   /// [references] can be empty if the comment does not contain any embedded
@@ -3278,6 +3285,7 @@ final class CommentImpl extends AstNodeImpl implements Comment {
     required this.tokens,
     required CommentType type,
     required List<CommentReferenceImpl> references,
+    required this.fencedCodeBlocks,
   }) : _type = type {
     _references._initialize(this, references);
   }
@@ -11942,6 +11950,45 @@ final class MapPatternImpl extends DartPatternImpl implements MapPattern {
   }
 }
 
+/// A Markdown fenced code block found in a documentation comment.
+@experimental
+final class MdFencedCodeBlock {
+  /// The 'info string'.
+  ///
+  /// This includes any text following the opening backticks. For example, in
+  /// a fenced code block starting with "```dart", the info string is "dart".
+  ///
+  /// If no text follows the opening backticks, the info string is `null`.
+  ///
+  /// See CommonMark specification at
+  /// <https://spec.commonmark.org/0.30/#fenced-code-blocks>.
+  final String? infoString;
+
+  /// Information about the comment lines that make up this code block.
+  final List<MdFencedCodeBlockLine> lines;
+
+  MdFencedCodeBlock({
+    required this.infoString,
+    required List<MdFencedCodeBlockLine> lines,
+  }) : lines = List.of(lines, growable: false);
+}
+
+/// A Markdown fenced code block line found in a documentation comment.
+@experimental
+final class MdFencedCodeBlockLine {
+  /// The offset of the start of the fenced code block, from the beginning of
+  /// compilation unit.
+  final int offset;
+
+  /// The length of the fenced code block.
+  final int length;
+
+  MdFencedCodeBlockLine({
+    required this.offset,
+    required this.length,
+  });
+}
+
 /// A method declaration.
 ///
 ///    methodDeclaration ::=