Add BuildBotClient to abstract http/logdog access

tools/gardening/lib/src/cache.dart

Index: tools/gardening/lib/src/cache.dart
diff --git a/tools/gardening/lib/src/cache.dart b/tools/gardening/lib/src/cache.dart
index 4dfac0128c5d714420b40bd549e2b68867862470..e9878bc0456624e1de82d9a66f3b93a2744f087a 100644
--- a/tools/gardening/lib/src/cache.dart
+++ b/tools/gardening/lib/src/cache.dart
@@ -23,6 +23,9 @@ class Cache {
       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()) {
@@ -39,6 +42,7 @@ class Cache {
 
   /// 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);