Reduce time spenty in tests: instead of looping for long, shorten optimizations threshold.

runtime/tests/vm/dart/byte_array_optimized_test.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
+// VMOptions=--optimization_counter_threshold=10
 
 // Library tag to be able to run in html test framework.
 library byte_array_test;
 
 import "package:expect/expect.dart";
 import 'dart:typed_data';
 
 // This test exercises optimized [] and []= operators
 // on byte array views.
 class OptimizedByteArrayTest {
@@ skipping 2108 matching lines @@
     testInt32ListView();
     testUint32ListView();
     testInt64ListView();
     testUint64ListView();
     testFloat32ListView();
     testFloat64ListView();
   }
 }
 
 main() {
-  for (var i=0; i<1000; i++) {
+  for (var i=0; i<20; i++) {
     OptimizedByteArrayTest.testMain();
   }
 }