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

tools/gardening/bin/bot.dart

+import 'dart:io';
+
+import 'compare_failures.dart' as compare_failures;
+import 'current_summary.dart' as current_summary;
+import 'status_summary.dart' as status_summary;
+
+void help(List<String> args) {
+  if (args.length == 1 && args[0] == "--help") {
+    print("This help");
+    return;
+  }
+
+  print("A script that combines multiple commands:\n");
+
+  commands.forEach((command, fun) {
+    print("$command:");
+    fun(["--help"]);
+    print("");
+  });
+}
+
+const commands = const {
+  "help": help,
+  "compare-failures": compare_failures.main,
+  "current-summary": current_summary.main,
+  "status-summary": status_summary.main,
+};
+
+void main(List<String> args) {
+  if (args.isEmpty) {
+    help([]);
+    exit(-1);
+  }
+  var command = commands[args[0]];
+  if (command == null) {
+    help([]);
+    exit(-1);
+  }
+  command(args.sublist(1));
+}