fixes #25610, infer JS builtin in strong mode like we used to

pkg/analyzer/test/src/task/strong/strong_test_helper.dart

Index: pkg/analyzer/test/src/task/strong/strong_test_helper.dart
diff --git a/pkg/analyzer/test/src/task/strong/strong_test_helper.dart b/pkg/analyzer/test/src/task/strong/strong_test_helper.dart
index f386377d44db4e985e227a86622e95901db2ce4d..5e346d05f3225abf9b0907a5b47e2b424bbd8983 100644
--- a/pkg/analyzer/test/src/task/strong/strong_test_helper.dart
+++ b/pkg/analyzer/test/src/task/strong/strong_test_helper.dart
@@ -118,6 +118,14 @@ final Map<String, String> mockSdkSources = {
         num/*=T*/ min/*<T extends num>*/(num/*=T*/ a, num/*=T*/ b) => null;
         num/*=T*/ max/*<T extends num>*/(num/*=T*/ a, num/*=T*/ b) => null;
         ''',
+
+  'dart:_foreign_helper': '''
+  library dart._foreign_helper;
+
+  JS(String typeDescription, String codeTemplate,
+    [arg0, arg1, arg2, arg3, arg4, arg5, arg6, arg7, arg8, arg9, arg10, arg11])
+  {}
+  '''
 };
 
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