Remove default heap size limit and add separate limit for externals.

tests/standalone/out_of_memory_test.dart

Index: tests/standalone/out_of_memory_test.dart
===================================================================
--- tests/standalone/out_of_memory_test.dart	(revision 43108)
+++ tests/standalone/out_of_memory_test.dart	(working copy)
@@ -1,6 +1,7 @@
 // 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=--old_gen_heap_size=512
 
 import "package:expect/expect.dart";