Use metadata annotation @patch for patch classes

runtime/bin/filter_patch.dart

Index: runtime/bin/filter_patch.dart
diff --git a/runtime/bin/filter_patch.dart b/runtime/bin/filter_patch.dart
index a5626bb3df026af2f5c7163f146c09c72ab06801..11691e863553b1da47a73a4bdfbae420c4bd01b9 100644
--- a/runtime/bin/filter_patch.dart
+++ b/runtime/bin/filter_patch.dart
@@ -28,16 +28,16 @@ class _ZLibDeflateFilter extends _FilterImpl {
       native "Filter_CreateZLibDeflate";
 }
 
-patch class _Filter {
-  /* patch */ static _Filter _newZLibDeflateFilter(bool gzip, int level,
-                                                   int windowBits, int memLevel,
-                                                   int strategy,
-                                                   List<int> dictionary,
-                                                   bool raw) =>
+@patch class _Filter {
+  /* @patch */ static _Filter _newZLibDeflateFilter(bool gzip, int level,
+                                                    int windowBits, int memLevel,
+                                                    int strategy,
+                                                    List<int> dictionary,
+                                                    bool raw) =>
       new _ZLibDeflateFilter(gzip, level, windowBits, memLevel, strategy,
                              dictionary, raw);
-  /* patch */ static _Filter _newZLibInflateFilter(int windowBits,
-                                                   List<int> dictionary,
-                                                   bool raw) =>
+  /* @patch */ static _Filter _newZLibInflateFilter(int windowBits,
+                                                    List<int> dictionary,
+                                                    bool raw) =>
       new _ZLibInflateFilter(windowBits, dictionary, raw);
 }