Get rid of join() and encapsulate File and Directory in io.dart.

utils/pub/hosted_source.dart

Index: utils/pub/hosted_source.dart
diff --git a/utils/pub/hosted_source.dart b/utils/pub/hosted_source.dart
index ef56462d37b9003fe078853b37c8621b624fb0b2..dda00a42c5197878751c998cbf6983f6cca42c97 100644
--- a/utils/pub/hosted_source.dart
+++ b/utils/pub/hosted_source.dart
@@ -11,6 +11,8 @@ import 'dart:uri';
 
 // TODO(nweiz): Make this import better.
 import '../../pkg/http/lib/http.dart' as http;
+import '../../pkg/path/lib/path.dart' as path;
+
 import 'http.dart';
 import 'io.dart';
 import 'log.dart' as log;
@@ -99,7 +101,7 @@ class HostedSource extends Source {
     });
 
     return new Future.immediate(
-        join(systemCacheRoot, urlDir, "${parsed.first}-${id.version}"));
+        path.join(systemCacheRoot, urlDir, "${parsed.first}-${id.version}"));
   }
 
   String packageName(description) => _parseDescription(description).first;