Better handling of large files.

dart/site/try/line_numbers.css

 /* Copyright (c) 2014, 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.
  */
 
 .mainEditorPane {
   padding: 0px;
-}
-
-div {
   counter-reset: dart-line-number;
 }
 
 .lineNumber {
   padding-left: 3.5em;
 }
 
 .lineNumber:before {
   counter-increment: dart-line-number;
   content: counter(dart-line-number) " ";
   position: absolute;
   left: 0px;
   width: 3em;
   text-align: right;
   background-color: lightgoldenrodyellow;
 }