Fix multi-isolate build:

src/platform-posix.cc

Index: src/platform-posix.cc
diff --git a/src/platform-posix.cc b/src/platform-posix.cc
index 9cf8b57e178304096b42fa70f3d1fddd19db9569..c4b0fb8226e79e98d2a1d30a4dd565b56b569e53 100644
--- a/src/platform-posix.cc
+++ b/src/platform-posix.cc
@@ -215,13 +215,14 @@ OS::MemCopyFunction CreateMemCopyFunction();
 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));