Add a SafepointMutexLocker class so that it is possible to have scopes inside the MutexLocker which…

runtime/vm/isolate.cc

Index: runtime/vm/isolate.cc
diff --git a/runtime/vm/isolate.cc b/runtime/vm/isolate.cc
index 27aa77b74f2503e2e1be72b6d40ecaf4628670db..8df0a8ac75c7b6775efc10fd5cd5591292005c94 100644
--- a/runtime/vm/isolate.cc
+++ b/runtime/vm/isolate.cc
@@ -801,6 +801,7 @@ Isolate::Isolate(const Dart_IsolateFlags& api_flags)
       random_(),
       simulator_(NULL),
       mutex_(new Mutex()),
+      symbols_mutex_(new Mutex()),
       saved_stack_limit_(0),
       deferred_interrupts_mask_(0),
       deferred_interrupts_(0),
@@ -863,6 +864,8 @@ Isolate::~Isolate() {
 #endif
   delete mutex_;
   mutex_ = NULL;  // Fail fast if interrupts are scheduled on a dead isolate.
+  delete symbols_mutex_;
+  symbols_mutex_ = NULL;
   delete message_handler_;
   message_handler_ = NULL;  // Fail fast if we send messages to a dead isolate.
   ASSERT(deopt_context_ == NULL);  // No deopt in progress when isolate deleted.