Add a 'bot.dart' executable to the gardening tools.

tools/gardening/bin/create_shard_groups.dart

 // Copyright (c) 2017, 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.
 
 // Outputs source code for `part` used by `shard2group.dart`. Usage:
 // `dart bin/create_shard_groups.dart source.html >lib/src/shard_data.dart`.
 
 import 'dart:io';
 import 'dart:math' as math;
 
@@ skipping 48 matching lines @@
       } else if (trimmed.contains(shardsGroupStartMark)) {
         // This is a group separator, switch to next sublist.
         if (shardResult != null) result.add(shardResult);
         shardResult = <String>[];
       }
     }
   }
   return result;
 }
 
+void help() {
+  print("Generates the 'shard_data.dart' file that is used by our gardening");
+  print("tools. This tool needs to be run when the buildbot configuration");
+  print("(shard) changes\n");
+  print('Usage: dart create_shard_groups.dart <darto-source-file>');
+}
+
 main(List<String> args) {
-  if (args.length != 1) {
-    print('Usage: dart create_shard_groups.dart <darto-source-file>');
+  if (args.length != 1 || args[0] == "--help") {
+    help();
+    if (args.length == 1) return;
     exit(1);
   }
 
   File dartoSourceFile = new File(args[0]);
   List<String> dartoSource = dartoSourceFile.readAsLinesSync();
 
   List<String> groups = findGroups(dartoSource);
   List<List<String>> shards = findShards(dartoSource);
   int groupCount = math.min(groups.length, shards.length);
 
@@ skipping 10 matching lines @@
 const Map<String, List<String>> shardGroups = const {""");
   for (int i = 0; i < groupCount; i++) {
     print("  '${groups[i]}': const <String>[");
     for (String shard in shards[i]) {
       print("    '$shard',");
     }
     print("  ],");
   }
   print('};');
 }