Add test to compare-failures

tools/gardening/test/test_client.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.
+
+/// Compares the test log of a build step with previous builds.
+///
+/// Use this to detect flakiness of failures, especially timeouts.
+
+import 'dart:async';
+import 'dart:io';
+
+import 'package:gardening/src/buildbot_loading.dart';
+import 'package:gardening/src/buildbot_structures.dart';
+import 'package:gardening/src/cache.dart';
+import 'package:gardening/src/client.dart';
+import 'package:gardening/src/util.dart';
+
+class TestClient implements BuildbotClient {
+  BuildbotClient _client;
+
+  /// Creates a mock client using logs stored in the `data` folder. If [force]
+  /// is `true`, missing logs are pulling from http and stored in the `data`
+  /// folder.
+  TestClient({bool force: false})
+      : _client = force ? new HttpBuildbotClient() : null;
+
+  String computePath(BuildUri buildUri) {
+    return 'data/${buildUri.botName}/${buildUri.buildNumber}'
+        '/${buildUri.stepName.replaceAll(' ', '_')}.log';
+  }
+
+  Future<String> readData(BuildUri buildUri) async {
+    String path = computePath(buildUri);
+    File file = new File.fromUri(Platform.script.resolve(path));
+    if (!file.existsSync() && _client != null) {
+      await file.parent.create();
+      log('Pulling $buildUri from http');
+      BuildResult result = await _client.readResult(buildUri);
+      if (result.buildNumber != null) {
+        print('Writing test data to $file');
+        String text = await cache.read(
+            result.buildUri.logdogPath,
+            () => throw new ArgumentError(
+                'Cache missing for ${result.buildUri.logdogPath}.'));
+        await file.writeAsString(text);
+      }
+    }
+    assert(file.existsSync(), "File $file not found.");
+    log('Reading test data from $file');
+    return file.readAsString();
+  }
+
+  @override
+  Future<BuildResult> readResult(BuildUri buildUri) async {
+    String text = await readData(buildUri);
+    return parseTestStepResult(buildUri, text);
+  }
+
+  @override
+  void close() {
+    _client?.close();
+  }
+
+  @override
+  int get mostRecentBuildNumber => -1;
+}