fix assert with message

sdk/lib/_internal/js_runtime/lib/js_helper.dart

Index: sdk/lib/_internal/js_runtime/lib/js_helper.dart
diff --git a/sdk/lib/_internal/js_runtime/lib/js_helper.dart b/sdk/lib/_internal/js_runtime/lib/js_helper.dart
index f46975eb8f9b96f2f413d4e776d7c46458a7c091..a80f0e243ae5db915c12811c786ca1ab66a4ada3 100644
--- a/sdk/lib/_internal/js_runtime/lib/js_helper.dart
+++ b/sdk/lib/_internal/js_runtime/lib/js_helper.dart
@@ -4049,10 +4049,9 @@ void mainHasTooManyParameters() {
 }
 
 class _AssertionError extends AssertionError {
-  final _message;
-  _AssertionError(this._message);
+  _AssertionError(Object message) : super(message);
 
-  String toString() => "Assertion failed: " + Error.safeToString(_message);
+  String toString() => "Assertion failed: " + Error.safeToString(message);
 }