Use metadata annotation @patch for patch classes

runtime/lib/symbol_patch.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // 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);
 
     // Remove closurization hash mark
     // #foo -> foo
     if (string.startsWith('#')) {
       string = string.substring(1);
     }
     // get:foo -> foo
@@ skipping 25 matching lines @@
       if (!skip) {
         result.write(char);
       }
     }
     if (add_setter_suffix) {
       result.write('=');
     }
     return result.toString();
   }
 
-  /* patch */ int get hashCode {
+  /* @patch */ int get hashCode {
     const arbitraryPrime = 664597;
     return 0x1fffffff & (arbitraryPrime * _name.hashCode);
   }
 }