Report the reason for a garbage collection when verbose GC is enabled.

runtime/vm/pages.h

 // Copyright (c) 2011, 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_PAGES_H_
 #define VM_PAGES_H_
 
 #include "vm/freelist.h"
 #include "vm/globals.h"
 #include "vm/virtual_memory.h"
@@ skipping 164 matching lines @@
   static bool IsPageAllocatableSize(intptr_t size) {
     return size <= kAllocatablePageSize;
   }
 
   void VisitObjects(ObjectVisitor* visitor) const;
   void VisitObjectPointers(ObjectPointerVisitor* visitor) const;
 
   RawObject* FindObject(FindObjectVisitor* visitor) const;
 
   // Collect the garbage in the page space using mark-sweep.
-  void MarkSweep(bool invoke_api_callbacks);
+  void MarkSweep(bool invoke_api_callbacks, const char* gc_reason);
 
   static HeapPage* PageFor(RawObject* raw_obj) {
     return reinterpret_cast<HeapPage*>(
         RawObject::ToAddr(raw_obj) & ~(kPageSize -1));
   }
 
   void StartEndAddress(uword* start, uword* end) const;
 
   void EnableGrowthControl() {
     page_space_controller_.Enable();
@@ skipping 48 matching lines @@
   bool sweeping_;
 
   PageSpaceController page_space_controller_;
 
   DISALLOW_IMPLICIT_CONSTRUCTORS(PageSpace);
 };
 
 }  // namespace dart
 
 #endif  // VM_PAGES_H_