Update implementation status of Mojo's dart:io

BUG=

Review URL: https://codereview.chromium.org/1759163002 .

mojo/dart/embedder/HACKING.md

diff --git a/mojo/dart/embedder/HACKING.md b/mojo/dart/embedder/HACKING.md
index f4f0f4a..4cc695c 100644
--- a/mojo/dart/embedder/HACKING.md
+++ b/mojo/dart/embedder/HACKING.md
@@ -58,4 +58,5 @@
 | SecureSocket       | N/A                  | No          |
 | SecureServerSocket | N/A                  | No          |
 | Datagram           | mojo:network_service | No          |
-| File system        | mojo:files           | No          |
+| File system        | mojo:files           | Yes         |
+| Zlib Gzip Filter   | N/A                  | Yes         |