Tolerate a comment with no end paragraph tag when generating preview

pkg/docgen/lib/docgen.dart

 // Copyright (c) 2013, 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.
 
 /// **docgen** is a tool for creating machine readable representations of Dart
 /// code metadata, including: classes, members, comments and annotations.
 ///
 /// docgen is run on a `.dart` file or a directory containing `.dart` files.
 ///
 ///      $ dart docgen.dart [OPTIONS] [FILE/DIR]
@@ skipping 920 matching lines @@
   }
 
   /// The type of this member to be used in index.txt.
   String get typeName => '';
 
   /// Creates a [Map] with this [Indexable]'s name and a preview comment.
   Map get previewMap {
     var finalMap = { 'name' : name, 'qualifiedName' : qualifiedName };
     if (comment != '') {
       var index = comment.indexOf('</p>');
-      finalMap['preview'] = '${comment.substring(0, index)}</p>';
+      finalMap['preview'] = index > 0 ?
+          '${comment.substring(0, index)}</p>' :
+          '<p><i>Comment preview not available</i></p>';
     }
     return finalMap;
   }
 
   /// Accessor to obtain the raw comment text for a given item, _without_ any
   /// of the links resolved.
   String get _commentText {
     String commentText;
     mirror.metadata.forEach((metadata) {
       if (metadata is CommentInstanceMirror) {
@@ skipping 1222 matching lines @@
         .map((e) => originalMirror.getField(e.simpleName).reflectee)
         .where((e) => e != null)
         .toList();
   }
 
   Map toMap() => {
     'name': Indexable.getDocgenObject(mirror, owningLibrary).docName,
     'parameters': parameters
   };
 }