Increase the allowed heap size for slow consumer test. We hit the limit in dartium x64 checked mode.

tests/lib/async/slow_consumer3_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=--old_gen_heap_size=32
+// VMOptions=--old_gen_heap_size=64
 
 library slow_consumer3_test;
 
 import 'dart:async';
 import 'dart:isolate';
 
 const int KB = 1024;
 const int MB = KB * KB;
 const int GB = KB * KB * KB;
 
@@ skipping 56 matching lines @@
   // file). If the consumer doesn't pause the data-provider it will run out of
   // heap-space.
 
   dataGenerator(100 * MB, 512 * KB)
     .pipe(new SlowConsumer(200 * MB, 3 * MB))
     .then((count) {
       port.close();
       Expect.equals(100 * MB, count);
     });
 }