Avoid infinities because they are not supported by JSON.

runtime/vm/pages.cc

 // 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.
 
 #include "vm/pages.h"
 
 #include "platform/assert.h"
 #include "vm/compiler_stats.h"
 #include "vm/gc_marker.h"
 #include "vm/gc_sweeper.h"
@@ skipping 367 matching lines @@
   JSONObject space(object, "old");
   space.AddProperty("type", "PageSpace");
   space.AddProperty("id", "heaps/old");
   space.AddProperty("name", "PageSpace");
   space.AddProperty("user_name", "old");
   space.AddProperty("collections", collections());
   space.AddProperty("used", UsedInWords() * kWordSize);
   space.AddProperty("capacity", CapacityInWords() * kWordSize);
   space.AddProperty("external", ExternalInWords() * kWordSize);
   space.AddProperty("time", MicrosecondsToSeconds(gc_time_micros()));
-  {
+  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);
   }
 }
 
 
 class HeapMapAsJSONVisitor : public ObjectVisitor {
  public:
   explicit HeapMapAsJSONVisitor(JSONArray* array)
       : ObjectVisitor(NULL), array_(array) {}
   virtual void VisitObject(RawObject* obj) {
     array_->AddValue(obj->Size() / kObjectAlignment);
@@ skipping 287 matching lines @@
     return 0;
   } else {
     ASSERT(total_time >= gc_time);
     int result= static_cast<int>((static_cast<double>(gc_time) /
                              static_cast<double>(total_time)) * 100);
     return result;
   }
 }
 
 }  // namespace dart