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

runtime/vm/pages.cc

 // 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.
 
 #include "vm/pages.h"
 
 #include "platform/assert.h"
 #include "vm/gc_marker.h"
 #include "vm/gc_sweeper.h"
 #include "vm/object.h"
@@ skipping 342 matching lines @@
     page = page->next();
   }
   page = large_pages_;
   while (page != NULL) {
     page->WriteProtect(read_only);
     page = page->next();
   }
 }
 
 
-void PageSpace::MarkSweep(bool invoke_api_callbacks) {
+void PageSpace::MarkSweep(bool invoke_api_callbacks, const char* reason) {
   // MarkSweep is not reentrant. Make sure that is the case.
   ASSERT(!sweeping_);
   sweeping_ = true;
   Isolate* isolate = Isolate::Current();
   NoHandleScope no_handles(isolate);
 
   if (FLAG_print_free_list_before_gc) {
     freelist_.Print();
   }
 
   if (FLAG_verify_before_gc) {
     OS::PrintErr("Verifying before MarkSweep...");
     heap_->Verify();
     OS::PrintErr(" done.\n");
   }
 
+  if (FLAG_verbose_gc) {
+    OS::PrintErr("Start mark sweep for %s collection\n", reason);
+  }
   Timer timer(true, "MarkSweep");
   timer.Start();
   int64_t start = OS::GetCurrentTimeMillis();
 
   // Mark all reachable old-gen objects.
   GCMarker marker(heap_);
   marker.MarkObjects(isolate, this, invoke_api_callbacks);
 
   // Reset the bump allocation page to unused.
   bump_page_ = NULL;
@@ skipping 177 matching lines @@
     return 0;
   } else {
     ASSERT(total_time >= gc_time);
     int result= static_cast<int>((static_cast<double>(gc_time) /
                              static_cast<double>(total_time)) * 100);
     return result;
   }
 }
 
 }  // namespace dart