[benckmark] Apply code review suggestion to TBR-ed ListCopy benchmark fix

Change-Id: I5a990bb21d4cc62e6ec6ff7428e126d3044b515a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/140721
Commit-Queue: Stephen Adams <sra@google.com>
Reviewed-by: Jonas Termansen <sortie@google.com>

benchmarks/ListCopy/dart/ListCopy.dart

@@ -57,7 +57,7 @@ class Benchmark extends BenchmarkBase {
       var variants = makeVariants();
       inputs.addAll(variants);
       totalLength +=
-          variants.fold(0, (sum, iterable) => sum + iterable.length) as int;
+          variants.fold<int>(0, (sum, iterable) => sum + iterable.length);
     }
 
     // Sanity checks.