Bunch of dartdoc changes.

utils/dartdoc/interact.dart

 // Copyright (c) 2011, 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.
 
 /** Provides client-side behavior for generated docs. */
 #library('interact');
 
 #import('dart:html');
 
 main() {
   window.on.contentLoaded.add((e) {
     for (var elem in document.queryAll('.method, .field')) {
       var showCode = elem.query('.show-code');
+
+      // Skip it if we don't have a code link. Will happen if source code is
+      // disabled.
+      if (showCode == null) continue;
+
       var pre = elem.query('pre.source');
       showCode.on.click.add((e) {
         if (pre.classes.contains('expanded')) {
           pre.classes.remove('expanded');
         } else {
           pre.classes.add('expanded');
         }
       });
     }
   });
 }