Reduce run-time of two longest running tests (checked mode).

tests/corelib/src/CollectionToStringTest.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_invocation_threshold=100
 
 /**
  * Tests for the toString methods on collections (including maps).
  */
 
 // TODO(jjb): seed random number generator when API allows it
 
-final int NUM_TESTS = 3000;
+final int NUM_TESTS = 300;
 final int MAX_COLLECTION_SIZE = 7;
 
 main() {
   smokeTest();
   exactTest();
   inexactTest();
 }
 
 
 /**
@@ skipping 266 matching lines @@
 bool randomBool() {
   return Math.random() < .5;
 }
 
 /** Returns the alphabetized characters in a string. */
 String alphagram(String s) {
   List<int> chars = s.charCodes();
   chars.sort((int a, int b) => a - b);
   return new String.fromCharCodes(chars);
 }