Add --single-threaded flag that disables background threads.

src/heap/mark-compact.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/v8.h"
 
 #include "src/base/atomicops.h"
 #include "src/base/bits.h"
 #include "src/code-stubs.h"
 #include "src/compilation-cache.h"
@@ skipping 443 matching lines @@
   }
 
   Heap* heap_;
   PagedSpace* space_;
 
   DISALLOW_COPY_AND_ASSIGN(SweeperTask);
 };
 
 
 void MarkCompactCollector::StartSweeperThreads() {
+  DCHECK(!FLAG_single_threaded);
   DCHECK(free_list_old_pointer_space_.get()->IsEmpty());
   DCHECK(free_list_old_data_space_.get()->IsEmpty());
   V8::GetCurrentPlatform()->CallOnBackgroundThread(
       new SweeperTask(heap(), heap()->old_data_space()),
       v8::Platform::kShortRunningTask);
   V8::GetCurrentPlatform()->CallOnBackgroundThread(
       new SweeperTask(heap(), heap()->old_pointer_space()),
       v8::Platform::kShortRunningTask);
 }
 
@@ skipping 3939 matching lines @@
   SlotsBuffer* buffer = *buffer_address;
   while (buffer != NULL) {
     SlotsBuffer* next_buffer = buffer->next();
     DeallocateBuffer(buffer);
     buffer = next_buffer;
   }
   *buffer_address = NULL;
 }
 }
 }  // namespace v8::internal