Fix a race condition in simulator that happens when flushing icache.

src/assembler.cc

Index: src/assembler.cc
diff --git a/src/assembler.cc b/src/assembler.cc
index 50ff603e7e4e727da98e26a47ae9b7d39d204230..e97b880ebc38dfabc49c87850047f82569e3be3a 100644
--- a/src/assembler.cc
+++ b/src/assembler.cc
@@ -190,6 +190,7 @@ void AssemblerBase::FlushICache(Isolate* isolate, void* start, size_t size) {
   if (size == 0) return;
 
 #if defined(USE_SIMULATOR)
+  base::LockGuard<base::Mutex> lock_guard(isolate->simulator_i_cache_mutex());
   Simulator::FlushICache(isolate->simulator_i_cache(), start, size);
 #else
   CpuFeatures::FlushICache(start, size);