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

tools/gardening/bin/compare_failures.dart

Index: tools/gardening/bin/compare_failures.dart
diff --git a/tools/gardening/bin/compare_failures.dart b/tools/gardening/bin/compare_failures.dart
index 8ccf5d66ce898ebf4e27ef016d073cd13e011397..ae85bac8815b92feda8b9da73077d7c5f2a59d94 100644
--- a/tools/gardening/bin/compare_failures.dart
+++ b/tools/gardening/bin/compare_failures.dart
@@ -14,15 +14,28 @@ import 'package:gardening/src/buildbot_structures.dart';
 import 'package:gardening/src/buildbot_loading.dart';
 import 'package:gardening/src/util.dart';
 
+void help(ArgParser argParser) {
+  print('Given a <log-uri> finds all failing tests in that stdout. Then ');
+  print('fetches earlier runs of the same bot and compares the results.');
+  print('This tool is particularly useful to detect flakes and their ');
+  print('frequency.');
+  print('Usage: compare_failures [options] <log-uri>');
+  print('where <log-uri> is the uri the stdio output of a failing test step');
+  print('and options are:');
+  print(argParser.usage);
+}
+
 main(List<String> args) async {
   ArgParser argParser = createArgParser();
+  argParser.addOption("run-count",
+      defaultsTo: "10", help: "How many previous runs should be fetched");
   ArgResults argResults = argParser.parse(args);
   processArgResults(argResults);
-  if (argResults.rest.length != 1) {
-    print('Usage: compare_failures [options] <log-uri>');
-    print('where <log-uri> is the uri the stdio output of a failing test step');
-    print('and options are:');
-    print(argParser.usage);
+  var runCount = int.parse(argResults['run-count'], onError: (_) => null);
+
+  if (argResults.rest.length != 1 || argResults['help'] || runCount == null) {
+    help(argParser);
+    if (argResults['help']) return;
     exit(1);
   }
   String url = argResults.rest.first;
@@ -33,7 +46,8 @@ main(List<String> args) async {
   Uri uri = Uri.parse(url);
   HttpClient client = new HttpClient();
   BuildUri buildUri = new BuildUri(uri);
-  List<BuildResult> results = await readBuildResults(client, buildUri);
+  List<BuildResult> results =
+      await readBuildResults(client, buildUri, runCount);
   print(generateBuildResultsSummary(buildUri, results));
   client.close();
 }
@@ -41,12 +55,12 @@ main(List<String> args) async {
 /// Creates a [BuildResult] for [buildUri] and, if it contains failures, the
 /// [BuildResult]s for the previous 5 builds.
 Future<List<BuildResult>> readBuildResults(
-    HttpClient client, BuildUri buildUri) async {
+    HttpClient client, BuildUri buildUri, int runCount) async {
   List<BuildResult> summaries = <BuildResult>[];
   BuildResult firstSummary = await readBuildResult(client, buildUri);
   summaries.add(firstSummary);
   if (firstSummary.hasFailures) {
-    for (int i = 0; i < 10; i++) {
+    for (int i = 0; i < runCount; i++) {
       buildUri = buildUri.prev();
       summaries.add(await readBuildResult(client, buildUri));
     }