Use metadata annotation @patch for patch classes

runtime/bin/platform_patch.dart

Index: runtime/bin/platform_patch.dart
diff --git a/runtime/bin/platform_patch.dart b/runtime/bin/platform_patch.dart
index 577bbb6dffcd67677533290438c93ccc362cf8a5..bbd353b4d078e1d2aa1daac0cbec92dbbb28a3c0 100644
--- a/runtime/bin/platform_patch.dart
+++ b/runtime/bin/platform_patch.dart
@@ -2,33 +2,33 @@
 // 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 _Platform {
-  /* patch */ static int _numberOfProcessors()
+@patch class _Platform {
+  /* @patch */ static int _numberOfProcessors()
       native "Platform_NumberOfProcessors";
-  /* patch */ static String _pathSeparator()
+  /* @patch */ static String _pathSeparator()
       native "Platform_PathSeparator";
-  /* patch */ static String _operatingSystem()
+  /* @patch */ static String _operatingSystem()
       native "Platform_OperatingSystem";
-  /* patch */ static _localHostname()
+  /* @patch */ static _localHostname()
       native "Platform_LocalHostname";
-  /* patch */ static _executable()
+  /* @patch */ static _executable()
       native "Platform_ExecutableName";
-  /* patch */ static _resolvedExecutable()
+  /* @patch */ static _resolvedExecutable()
       native "Platform_ResolvedExecutableName";
-  /* patch */ static _environment()
+  /* @patch */ static _environment()
       native "Platform_Environment";
-  /* patch */ static List<String> _executableArguments()
+  /* @patch */ static List<String> _executableArguments()
       native "Platform_ExecutableArguments";
-  /* patch */ static String _version()
+  /* @patch */ static String _version()
       native "Platform_GetVersion";
 
-  /* patch */ static String _packageRoot()
+  /* @patch */ static String _packageRoot()
       => VMLibraryHooks.packageRootString;
-  /* patch */ static String _packageConfig()
+  /* @patch */ static String _packageConfig()
       => VMLibraryHooks.packageConfigString;
 
   // This script singleton is written to by the embedder if applicable.
-  /* patch */ static void set _nativeScript(String path) {
+  /* @patch */ static void set _nativeScript(String path) {
     if (path.startsWith('http:') ||
         path.startsWith('https:') ||
         path.startsWith('package:') ||