Remove the Path class from dart:io

sdk/lib/_internal/dartdoc/lib/dartdoc.dart

Index: sdk/lib/_internal/dartdoc/lib/dartdoc.dart
diff --git a/sdk/lib/_internal/dartdoc/lib/dartdoc.dart b/sdk/lib/_internal/dartdoc/lib/dartdoc.dart
index 11ef3374cb00a54959b927759ec9c5b37349eb59..f005351da8cae00cb969e400e910b7e5806b781d 100644
--- a/sdk/lib/_internal/dartdoc/lib/dartdoc.dart
+++ b/sdk/lib/_internal/dartdoc/lib/dartdoc.dart
@@ -2128,15 +2128,15 @@ class Dartdoc {
     write("# VERSION: ${new DateTime.now()}\n\n");
     write("NETWORK:\n*\n\n");
     write("CACHE:\n");
-    var toCache = new Directory.fromPath(outputDir);
+    var toCache = new Directory(outputDir);
     toCache.list(recursive: true).listen(
         (FileSystemEntity entity) {
-          if (entity.isFile) {
+          if (entity is File) {
             var filename = entity.path;
             if (filename.endsWith('appcache.manifest')) {
               return;
             }
-            Path relativeFilePath = new Path(filename).relativeTo(outputDir);
+            String relativeFilePath = path.relative(filename, from: outputDir);
             write("$relativeFilePath\n");
           }
         },