Avoid infinities because they are not supported by JSON.

runtime/vm/scavenger.cc

Index: runtime/vm/scavenger.cc
diff --git a/runtime/vm/scavenger.cc b/runtime/vm/scavenger.cc
index c6a3e3e7aa566f3c6b7a03cefd48368abf797b86..e40c5a6ddfaaf54dee25aae10c048d558babe7fc 100644
--- a/runtime/vm/scavenger.cc
+++ b/runtime/vm/scavenger.cc
@@ -844,7 +844,7 @@ void Scavenger::PrintToJSONObject(JSONObject* object) {
   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);
@@ -852,6 +852,8 @@ void Scavenger::PrintToJSONObject(JSONObject* object) {
         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);