Use io.dirExists instead of Directory.existsSync for consistency

Author: Jesse Bakker

lib/src/system_cache.dart

@@ -6,8 +6,7 @@ import 'dart:io';
 
 import 'package:path/path.dart' as p;
 
-import 'io.dart';
-import 'io.dart' as io show createTempDir;
+import 'io.dart' as io show createTempDir, dirExists;
 import 'log.dart' as log;
 import 'package.dart';
 import 'package_name.dart';
@@ -38,7 +37,7 @@ class SystemCache {
       // If a cache dir already exists in %APPDATA%, use it, else default to %LOCALAPPDATA%
       var appData = Platform.environment['APPDATA'];
       var appDataCacheDir = p.join(appData, 'Pub', 'Cache');
-      if (Directory(appDataCacheDir).existsSync()) {
+      if (io.dirExists(appDataCacheDir)) {
         return appDataCacheDir;
       }
       var localAppData = Platform.environment['LOCALAPPDATA'];