Fix old dart2js patch reference in service extensions

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 3dd4f665babb211bce84cc35ddb88c640a8da0ab..69dc37975f8dc5737674916374f3863c603392dd 100644
--- a/sdk/lib/_internal/js_runtime/lib/developer_patch.dart
+++ b/sdk/lib/_internal/js_runtime/lib/developer_patch.dart
@@ -36,8 +36,8 @@ void log(String message,
 final _extensions = new Map<String, ServiceExtensionHandler>();
 
 @patch
-bool _extensionExists(String method) {
-  return _extensions[method] != null;
+ServiceExtensionHandler _lookupExtension(String method);
+  return _extensions[method];
 }
 
 @patch