Add VMService GC events

runtime/vm/heap.h

 // 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.
 
 #ifndef VM_HEAP_H_
 #define VM_HEAP_H_
 
 #include "platform/assert.h"
 #include "vm/allocation.h"
 #include "vm/flags.h"
@@ skipping 306 matching lines @@
 
   // GC stats collection.
   GCStats stats_;
 
   // This heap is in read-only mode: No allocation is allowed.
   bool read_only_;
 
   // GC on the heap is in progress.
   bool gc_in_progress_;
 
+  friend class GCEvent;
   friend class GCTestHelper;
   DISALLOW_COPY_AND_ASSIGN(Heap);
 };
 
 
+class GCEvent {
+ public:
+  explicit GCEvent(const Heap::GCStats& stats)
+      : stats_(stats) {}
+  void PrintJSON(JSONStream* js) const;
+ private:
+  const Heap::GCStats& stats_;
+};
+
+
 #if defined(DEBUG)
 class NoGCScope : public StackResource {
  public:
   NoGCScope();
   ~NoGCScope();
  private:
   DISALLOW_COPY_AND_ASSIGN(NoGCScope);
 };
 #else  // defined(DEBUG)
 class NoGCScope : public ValueObject {
@@ skipping 10 matching lines @@
   NoHeapGrowthControlScope();
   ~NoHeapGrowthControlScope();
  private:
   bool current_growth_controller_state_;
   DISALLOW_COPY_AND_ASSIGN(NoHeapGrowthControlScope);
 };
 
 }  // namespace dart
 
 #endif  // VM_HEAP_H_