[gardening] Fix gen_kernel path on Windows for product_aot_kernel_test.dart.

This is follow-up to https://dart-review.googlesource.com/c/sdk/+/127004.

Change-Id: I2ccdfee835beb5182a73ca065dae1791e526295f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/127380
Reviewed-by: Alexander Aprelev <[email protected]>
Commit-Queue: Alexander Aprelev <[email protected]>

Author: aam

runtime/tests/vm/dart/product_aot_kernel_test.dart

@@ -36,7 +36,9 @@ Future main(List<String> args) async {
 
     // Compile script to Kernel IR.
     await File(helloFile).writeAsString('main() => print("Hello");');
-    await run('pkg/vm/tool/gen_kernel', <String>[
+    final genKernel = Platform.isWindows ?
+      "pkg\\vm\\tool\\gen_kernel.bat" : 'pkg/vm/tool/gen_kernel';
+    await run(genKernel, <String>[
       '--aot',
       '--platform=$platformDill',
       '-o',