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

tests/language/src/MathVMTest.dart

Index: tests/language/src/MathVMTest.dart
===================================================================
--- tests/language/src/MathVMTest.dart	(revision 4470)
+++ tests/language/src/MathVMTest.dart	(working copy)
@@ -2,6 +2,7 @@
 // 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.
 // Tests that the VM does not crash on weird corner cases of class Math.
+// VMOptions=--optimization_invocation_threshold=100
 
 class FakeNumber {
   const FakeNumber();
@@ -33,7 +34,7 @@
   }
 }
 main() {
-  for (int i = 0; i < 2000; i++) {
+  for (int i = 0; i < 200; i++) {
     MathTest.testMain();
   }
 }