Reduce number of iterations for the regexp-cache test.

tests/corelib_2/reg_exp_cache_test.dart

 // Copyright (c) 2017, 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.
 
 // Runs several similar regexps in a loop to see if an internal cache works (at
 // least in easy conditions).
 
 import "package:expect/expect.dart";
 
 void main() {
-  for (int j = 1; j < 50; j++) {
-    for (int i = 0; i < 20 * j; i++) {
+  for (int j = 1; j < 50; j += 4) {
+    for (int i = 0; i < 15 * j; i++) {
       var regExp = new RegExp("foo$i");
       var match = regExp.firstMatch("foo$i");
       Expect.isNotNull(match);
       Expect.equals("foo$i", match[0]);
     }
   }
 }