Implemented an averaging object histogram in the vm.

runtime/vm/object.cc

Index: runtime/vm/object.cc
===================================================================
--- runtime/vm/object.cc	(revision 23906)
+++ runtime/vm/object.cc	(working copy)
@@ -390,7 +390,7 @@
         Class::kNoTypeArguments;
     cls.raw_ptr()->num_native_fields_ = 0;
     cls.InitEmptyFields();
-    isolate->class_table()->Register(cls);
+    isolate->RegisterClass(cls);
   }
 
   // Allocate and initialize the null class.
@@ -1468,7 +1468,7 @@
   result.raw_ptr()->num_native_fields_ = 0;
   result.raw_ptr()->token_pos_ = Scanner::kDummyTokenIndex;
   result.InitEmptyFields();
-  Isolate::Current()->class_table()->Register(result);
+  Isolate::Current()->RegisterClass(result);
   return result.raw();
 }
 
@@ -1879,7 +1879,7 @@
   result.raw_ptr()->num_native_fields_ = 0;
   result.raw_ptr()->token_pos_ = Scanner::kDummyTokenIndex;
   result.InitEmptyFields();
-  Isolate::Current()->class_table()->Register(result);
+  Isolate::Current()->RegisterClass(result);
   return result.raw();
 }