Avoid infinities because they are not supported by JSON.

runtime/vm/scavenger.cc

 // Copyright (c) 2011, 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.
 
 #include "vm/scavenger.h"
 
 #include <algorithm>
 #include <map>
 #include <utility>
 
@@ skipping 826 matching lines @@
 
 void Scavenger::PrintToJSONObject(JSONObject* object) {
   Isolate* isolate = Isolate::Current();
   ASSERT(isolate != NULL);
   JSONObject space(object, "new");
   space.AddProperty("type", "Scavenger");
   space.AddProperty("id", "heaps/new");
   space.AddProperty("name", "Scavenger");
   space.AddProperty("user_name", "new");
   space.AddProperty("collections", collections());
-  {
+  if (collections() > 0) {
     int64_t run_time = OS::GetCurrentTimeMicros() - isolate->start_time();
     run_time = Utils::Maximum(run_time, static_cast<int64_t>(0));
     double run_time_millis = MicrosecondsToMilliseconds(run_time);
     double avg_time_between_collections =
         run_time_millis / static_cast<double>(collections());
     space.AddProperty("avgCollectionPeriodMillis",
                       avg_time_between_collections);
+  } else {
+    space.AddProperty("avgCollectionPeriodMillis", 0.0);
   }
   space.AddProperty("used", UsedInWords() * kWordSize);
   space.AddProperty("capacity", CapacityInWords() * kWordSize);
   space.AddProperty("external", ExternalInWords() * kWordSize);
   space.AddProperty("time", MicrosecondsToSeconds(gc_time_micros()));
 }
 
 
 void Scavenger::AllocateExternal(intptr_t size) {
   ASSERT(size >= 0);
   external_size_ += size;
 }
 
 
 void Scavenger::FreeExternal(intptr_t size) {
   ASSERT(size >= 0);
   external_size_ -= size;
   ASSERT(external_size_ >= 0);
 }
 
 }  // namespace dart