[gardening] Fix generated vm/dart/generated/many_double_literals_test

The mistake in the test generation results in a huge number of syntax
errors which cause timeouts and infra failures.

TEST=vm/dart/generated/many_double_literals_test

Change-Id: Ie21d5249b580b07af25ab5e498877f657eb0a9bd
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/445240
Reviewed-by: Ben Konyi <[email protected]>
Commit-Queue: Alexander Markov <[email protected]>

Author: alexmarkov

runtime/tests/vm/dart/generated/many_double_literals_generator.dart

@@ -14,7 +14,7 @@ main() {
   for (var i = 0; i < m; i++) {
     print("chunk${i}() {");
     for (var j = 0; j < n / m; j++) {
-      print("  add(${n / m * i + j}.0);");
+      print("  add(${n ~/ m * i + j}.0);");
     }
     print("}");
   }