Tolerate a comment with no end paragraph tag when generating preview

pkg/docgen/lib/docgen.dart

Index: pkg/docgen/lib/docgen.dart
diff --git a/pkg/docgen/lib/docgen.dart b/pkg/docgen/lib/docgen.dart
index d316780fd4e16c639c3e3d74768e2278cbdca542..a9f8848b65194b3c3d47cc65e0b3be36b1a3a380 100644
--- a/pkg/docgen/lib/docgen.dart
+++ b/pkg/docgen/lib/docgen.dart
@@ -938,7 +938,9 @@ abstract class Indexable extends MirrorBased {
     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;
   }