Use metadata annotation @patch for patch classes

runtime/lib/double_patch.dart

Index: runtime/lib/double_patch.dart
diff --git a/runtime/lib/double_patch.dart b/runtime/lib/double_patch.dart
index f33122ac763da4de7ff8611918fa8aa8d5c5a204..cc3c6054e0c3923914b6e196d9f85e89542e777c 100644
--- a/runtime/lib/double_patch.dart
+++ b/runtime/lib/double_patch.dart
@@ -5,7 +5,7 @@
 
 // VM implementation of double.
 
-patch class double {
+@patch class double {
 
   static double _nativeParse(String str,
                              int start, int end) native "Double_parse";
@@ -101,8 +101,8 @@ patch class double {
     return _nativeParse(str, start, end);
   }
 
-  /* patch */ static double parse(String str,
-                                  [double onError(String str)]) {
+  /* @patch */ static double parse(String str,
+                                   [double onError(String str)]) {
     var result = _parse(str);
     if (result == null) {
       if (onError == null) throw new FormatException("Invalid double", str);