Unparse comment references using AST tokens (#2642)

* Unparse comment references via tokens instead of file access

* dartfmt

* Rewording

lib/src/comment_references/model_comment_reference.dart

@@ -6,7 +6,6 @@ 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 {
   /// Does the structure of the reference itself imply a possible default
@@ -70,9 +69,15 @@ class _ModelCommentReferenceImpl implements ModelCommentReference {
   /// [CommentReference].
   static String _referenceText(
       CommentReference ref, ResourceProvider resourceProvider) {
-    var contents = getFileContentsFor(
-        (ref.root as CompilationUnit).declaredElement, resourceProvider);
-    return contents.substring(ref.offset, ref.end);
+    var token = (ref.parent as Comment)
+        .tokens
+        .firstWhere((t) => t.offset <= ref.offset && t.end >= ref.end);
+    // This is a little sketchy, but works since comments happen to be a token
+    // that is fully preserved in its string representation.
+    // TODO(jcollins-g): replace unparsing in general with lower level changes.
+    return token
+        .toString()
+        .substring(ref.offset - token.offset, ref.end - token.offset);
   }
 
   List<CommentReferenceNode> _parsed;

lib/src/model_utils.dart

@@ -96,6 +96,12 @@ Iterable<Class> findCanonicalFor(Iterable<Class> classes) {
       c.packageGraph.findCanonicalModelElementFor(c.element) as Class ?? c);
 }
 
+/// Uses direct file access to get the contents of a file.  Cached.
+///
+/// Direct reading of source code via a [PhysicalResourceProvider] is not
+/// allowed in some environments, so avoid using this.
+// TODO(jcollins-g): consider deprecating this and the `--include-source`
+// feature that uses it now that source code linking is possible.
 String getFileContentsFor(Element e, ResourceProvider resourceProvider) {
   var location = e.source.fullName;
   if (!_fileContents.containsKey(location)) {