Add BuildBotClient to abstract http/logdog access

tools/gardening/lib/src/cache.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.
 
 import 'dart:async';
 import 'dart:io';
 
 import 'util.dart';
 
 final Cache cache = new Cache(Uri.base.resolve('temp/gardening-cache/'));
 
 /// Simple cache for test step output.
 class Cache {
   Uri base;
 
   Cache(this.base);
 
   Map<String, String> memoryCache = <String, String>{};
 
   /// Load the cache text for [path] or call [ifAbsent] to fetch the data.
   Future<String> read(String path, Future<String> ifAbsent()) async {
     if (memoryCache.containsKey(path)) {
       log('Found $path in memory cache');
       return memoryCache[path];
     }
+    if (base == null) {
+      return ifAbsent();
+    }
     File file = new File.fromUri(base.resolve(path));
     String text;
     if (file.existsSync()) {
       log('Found $path in file cache');
       text = file.readAsStringSync();
       memoryCache[path] = text;
     } else {
       log('Loading $path');
       text = await ifAbsent();
       write(path, text);
     }
     return text;
   }
 
   /// Store [text] as the cache data for [path].
   void write(String path, String text) {
+    if (base == null) return;
     log('Creating $path in file cache');
     File file = new File.fromUri(base.resolve(path));
     file.createSync(recursive: true);
     file.writeAsStringSync(text);
     memoryCache[path] = text;
   }
 }