Light-weight stats counters for experiments.

runtime/vm/isolate.h

Index: runtime/vm/isolate.h
===================================================================
--- runtime/vm/isolate.h	(revision 37279)
+++ runtime/vm/isolate.h	(working copy)
@@ -10,6 +10,7 @@
 #include "platform/thread.h"
 #include "vm/base_isolate.h"
 #include "vm/class_table.h"
+#include "vm/counters.h"
 #include "vm/handles.h"
 #include "vm/megamorphic_cache_table.h"
 #include "vm/random.h"
@@ -542,6 +543,8 @@
 
   static void VisitIsolates(IsolateVisitor* visitor);
 
+  Counters* counters() { return &counters_; }
+
  private:
   Isolate();
 
@@ -617,6 +620,8 @@
   RawGrowableObjectArray* tag_table_;
   RawUserTag* current_tag_;
 
+  Counters counters_;
+
   // Isolate list next pointer.
   Isolate* next_;