Use metadata annotation @patch for patch classes

runtime/lib/stopwatch_patch.dart

Index: runtime/lib/stopwatch_patch.dart
diff --git a/runtime/lib/stopwatch_patch.dart b/runtime/lib/stopwatch_patch.dart
index e16005cab8652c6477803310a6e76e47eca74406..e3749e848c0742effabbe11c2c7f99a167fc4b7d 100644
--- a/runtime/lib/stopwatch_patch.dart
+++ b/runtime/lib/stopwatch_patch.dart
@@ -4,15 +4,15 @@
 
 // A VM patch of the stopwatch part of dart:core.
 
-patch class Stopwatch {
-  /* patch */ static void _initTicker() {
+@patch class Stopwatch {
+  /* @patch */ static void _initTicker() {
     if (_frequency == null) {
       _frequency = _computeFrequency();
     }
   }
 
   // Returns the current clock tick.
-  /* patch */ static int _now() native "Stopwatch_now";
+  /* @patch */ static int _now() native "Stopwatch_now";
 
   // Returns the frequency of clock ticks in Hz.
   static int _computeFrequency() native "Stopwatch_frequency";