Fix multi-isolate build:

src/platform-posix.cc

 // Copyright 2011 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 197 matching lines @@
 #if defined(V8_TARGET_ARCH_IA32)
 static OS::MemCopyFunction memcopy_function = NULL;
 static Mutex* memcopy_function_mutex = OS::CreateMutex();
 // Defined in codegen-ia32.cc.
 OS::MemCopyFunction CreateMemCopyFunction();
 
 // Copy memory area to disjoint memory area.
 void OS::MemCopy(void* dest, const void* src, size_t size) {
   if (memcopy_function == NULL) {
     ScopedLock lock(memcopy_function_mutex);
-    Isolate::EnsureDefaultIsolate();
     if (memcopy_function == NULL) {
       OS::MemCopyFunction temp = CreateMemCopyFunction();
       MemoryBarrier();
       memcopy_function = temp;
     }
   }
+  // Note: here we rely on dependent reads being ordered. This is true
+  // on all architectures we currently support.
   (*memcopy_function)(dest, src, size);
 #ifdef DEBUG
   CHECK_EQ(0, memcmp(dest, src, size));
 #endif
 }
 #endif  // V8_TARGET_ARCH_IA32
 
 // ----------------------------------------------------------------------------
 // POSIX string support.
 //
@@ skipping 179 matching lines @@
   return ntohl(value);
 }
 
 
 Socket* OS::CreateSocket() {
   return new POSIXSocket();
 }
 
 
 } }  // namespace v8::internal