Write basic parser for comment references (#2634)

* Write a basic parser for comment references

* Add some simpler cases

* review comments

Author: jcollins-g

lib/src/comment_references/model_comment_reference.dart

@@ -5,19 +5,39 @@
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/file_system/file_system.dart';
+import 'package:dartdoc/src/comment_references/parser.dart';
 import 'package:dartdoc/src/model_utils.dart';
 
+abstract class ModelCommentReference {
+  String get codeRef;
+  Element get staticElement;
+
+  /// Construct a [ModelCommentReference] using the analyzer AST.
+  factory ModelCommentReference(
+          CommentReference ref, ResourceProvider resourceProvider) =>
+      _ModelCommentReferenceImpl(ref, resourceProvider);
+
+  /// Construct a [ModelCommentReference] given a raw string.
+  factory ModelCommentReference.synthetic(String codeRef, Element element) =>
+      _ModelCommentReferenceImpl.synthetic(codeRef, element);
+}
+
 /// A stripped down analyzer AST [CommentReference] containing only that
 /// information needed for Dartdoc.  Drops link to the [CommentReference]
 /// and [ResourceProvider] after construction.
-class ModelCommentReference {
+class _ModelCommentReferenceImpl implements ModelCommentReference {
+  @override
   final String codeRef;
+  @override
   final Element staticElement;
 
-  ModelCommentReference(CommentReference ref, ResourceProvider resourceProvider)
+  _ModelCommentReferenceImpl(
+      CommentReference ref, ResourceProvider resourceProvider)
       : codeRef = _referenceText(ref, resourceProvider),
         staticElement = ref.identifier.staticElement;
 
+  _ModelCommentReferenceImpl.synthetic(this.codeRef, this.staticElement);
+
   /// "Unparse" the code reference into the raw text associated with the
   /// [CommentReference].
   static String _referenceText(
@@ -26,4 +46,6 @@ class ModelCommentReference {
         (ref.root as CompilationUnit).declaredElement, resourceProvider);
     return contents.substring(ref.offset, ref.end);
   }
+
+  List<CommentReferenceNode> parse() => CommentReferenceParser(codeRef).parse();
 }