Implemented an averaging object histogram in the vm.

runtime/vm/heap_histogram.h

Index: runtime/vm/heap_histogram.h
===================================================================
--- runtime/vm/heap_histogram.h	(revision 0)
+++ runtime/vm/heap_histogram.h	(revision 0)
@@ -0,0 +1,68 @@
+// Copyright (c) 2013, 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.
+
+#ifndef VM_HEAP_HISTOGRAM_H_
+#define VM_HEAP_HISTOGRAM_H_
+
+#include "platform/assert.h"
+#include "vm/flags.h"
+#include "vm/globals.h"
+#include "vm/raw_object.h"
+
+namespace dart {
+
+DECLARE_FLAG(bool, print_object_histogram);
+
+// ObjectHistogram is used to compute an average object histogram over
+// the lifetime of an isolate and then print the histogram when the isolate
+// is shut down. Information is gathered at the back-edge of each major GC
+// event. When an object histogram is collected for an isolate, an extra major
+// GC is performed just prior to shutdown.
+class ObjectHistogram {
+ public:
+  explicit ObjectHistogram(Isolate* isolate);
+  ~ObjectHistogram();
+
+  // Called when a new class is registered in the isolate.
+  void RegisterClass(const Class& cls);
+
+  // Collect sample for the histogram. Called at back-edge of major GC.
+  void Collect();
+
+  // Print the histogram on stdout.
+  void Print();
+
+ private:
+  // Add obj to histogram
+  void Add(RawObject* obj);
+
+  // For each class an Element keeps track of the accounting.
+  class Element : public ValueObject {
+   public:
+    void Add(int size) {
+      count_++;
+      size_ += size;
+    }
+    intptr_t class_id_;
+    intptr_t count_;
+    intptr_t size_;
+  };
+
+  // Compare function for sorting result.
+  static int compare(const Element** a, const Element** b);
+
+  intptr_t major_gc_count_;
+  intptr_t table_length_;
+  Element* table_;
+  Isolate* isolate_;
+
+  friend class ObjectHistogramVisitor;
+
+  DISALLOW_COPY_AND_ASSIGN(ObjectHistogram);
+};
+
+}  // namespace dart
+
+#endif  // VM_HEAP_HISTOGRAM_H_
+