Use metadata annotation @patch for patch classes

runtime/lib/symbol_patch.dart

Index: runtime/lib/symbol_patch.dart
diff --git a/runtime/lib/symbol_patch.dart b/runtime/lib/symbol_patch.dart
index 1a1b2bcde7f654a1dcd94a6f50b8903775dacc52..4c70daa1e95b3bcdfd7358b1ec89a5a3ee67009e 100644
--- a/runtime/lib/symbol_patch.dart
+++ b/runtime/lib/symbol_patch.dart
@@ -2,11 +2,11 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-patch class Symbol {
-  /* patch */ const Symbol(String name)
+@patch class Symbol {
+  /* @patch */ const Symbol(String name)
       : this._name = name;
 
-  /* patch */ toString() => 'Symbol("${getUnmangledName(this)}")';
+  /* @patch */ toString() => 'Symbol("${getUnmangledName(this)}")';
 
   static getUnmangledName(Symbol symbol) {
     String string = Symbol.getName(symbol);
@@ -52,7 +52,7 @@ patch class Symbol {
     return result.toString();
   }
 
-  /* patch */ int get hashCode {
+  /* @patch */ int get hashCode {
     const arbitraryPrime = 664597;
     return 0x1fffffff & (arbitraryPrime * _name.hashCode);
   }