Use metadata for patching.

sdk/lib/_internal/lib/internal_patch.dart

Index: sdk/lib/_internal/lib/internal_patch.dart
diff --git a/sdk/lib/_internal/lib/internal_patch.dart b/sdk/lib/_internal/lib/internal_patch.dart
index 485c8956de4fc8e34a10d138e0906fc50f142893..16afd5f3aa6332ec62190bdf31b5ff3455f4f898 100644
--- a/sdk/lib/_internal/lib/internal_patch.dart
+++ b/sdk/lib/_internal/lib/internal_patch.dart
@@ -3,19 +3,23 @@
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:_js_primitives' show printString;
-import 'dart:_js_helper' show JS;
+import 'dart:_js_helper' show JS, patch;
 import 'dart:_interceptors' show JSArray;
 
-patch class Symbol implements core.Symbol {
-  patch const Symbol(String name)
+@patch
+class Symbol implements core.Symbol {
+  @patch
+  const Symbol(String name)
       : this._name = name;
 }
 
-patch void printToConsole(String line) {
+@patch
+void printToConsole(String line) {
   printString('$line');
 }
 
-patch List makeListFixedLength(List growableList) {
+@patch
+List makeListFixedLength(List growableList) {
   JSArray.markFixedList(growableList);
   return growableList;
 }