Fix doc index generation to include `error` rules.

tool/doc.dart

 // Copyright (c) 2015, 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.
 
 import 'dart:io';
 
 import 'package:args/args.dart';
 import 'package:linter/src/linter.dart';
 import 'package:linter/src/rules.dart';
 import 'package:markdown/markdown.dart';
@@ skipping 37 matching lines @@
 These rules are under active development.  Feedback is 
 [welcome](https://github.com/dart-lang/linter/issues)!
 ''';
 
 const ruleLeadMatter = 'Rules are organized into familiar rule groups.';
 
 /// Sorted list of contributed lint rules.
 final List<LintRule> rules =
     new List<LintRule>.from(ruleRegistry, growable: false)..sort();
 
-String get enumeratePubRules => rules
-    .where((r) => r.group == Group.pub)
+String get enumerateErrorRules => rules
+    .where((r) => r.group == Group.errors)
     .map((r) => '${toDescription(r)}')
     .join('\n\n');
 
 String get enumerateGroups => Group.builtin
     .map((Group g) =>
         '<li><strong>${g.name}</strong> - ${markdownToHtml(g.description)}</li>')
     .join('\n');
 
+String get enumeratePubRules => rules
+    .where((r) => r.group == Group.pub)
+    .map((r) => '${toDescription(r)}')
+    .join('\n\n');
+
 String get enumerateStyleRules => rules
     .where((r) => r.group == Group.style)
     .map((r) => '${toDescription(r)}')
     .join('\n\n');
 
 void printUsage(ArgParser parser, [String error]) {
   var message = 'Generates lint docs.';
   if (error != null) {
     message = error;
   }
@@ skipping 127 matching lines @@
             </p>
             <ul>
                <p>
                   $enumerateGroups
                </p>
             </ul>
             <p>
                ${markdownToHtml(ruleFootMatter)}
             </p>
 
+            <h2 id="styleguide-rules">Error Rules</h2>
+
+               $enumerateErrorRules
+
             <h2 id="styleguide-rules">Style Rules</h2>
 
                $enumerateStyleRules
 
             <h2 id="styleguide-rules">Pub Rules</h2>
 
                $enumeratePubRules
             
          </section>
       </div>
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 ''';
 }