Re-enable the benchmark test.

Bug:
Change-Id: I433bb1a0944a8b1bf12400c62d97246031a73082
Reviewed-on: https://dart-review.googlesource.com/28843
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Devon Carew <devoncarew@google.com>

pkg/analysis_server/test/benchmarks_test.dart

@@ -53,7 +53,7 @@ void defineTests() {
         );
         expect(r.exitCode, 0,
             reason: 'exit: ${r.exitCode}\n${r.stdout}\n${r.stderr}');
-      }, skip: true); // #31554
+      });
     }
   });
 }