Rework service extensions to be safe

sdk/lib/_internal/js_runtime/lib/developer_patch.dart

Index: sdk/lib/_internal/js_runtime/lib/developer_patch.dart
diff --git a/sdk/lib/_internal/js_runtime/lib/developer_patch.dart b/sdk/lib/_internal/js_runtime/lib/developer_patch.dart
index 2b5ba7ea055e050566d4a8f8b27357cf909efd57..3dd4f665babb211bce84cc35ddb88c640a8da0ab 100644
--- a/sdk/lib/_internal/js_runtime/lib/developer_patch.dart
+++ b/sdk/lib/_internal/js_runtime/lib/developer_patch.dart
@@ -32,3 +32,15 @@ void log(String message,
           StackTrace stackTrace}) {
   // TODO.
 }
+
+final _extensions = new Map<String, ServiceExtensionHandler>();
+
+@patch
+bool _extensionExists(String method) {
+  return _extensions[method] != null;
+}
+
+@patch
+void _registerExtension(String method, ServiceExtensionHandler handler) {
+  _extensions[method] = handler;
+}