Separate mutex for megamorphic cache tables lookup in the background compiler.

runtime/vm/isolate.cc

Index: runtime/vm/isolate.cc
diff --git a/runtime/vm/isolate.cc b/runtime/vm/isolate.cc
index bd26039f1e8b524f271d2dc1c6a53b41cff88847..88fa1b81169772346b5476f028b7097a9ddb79e0 100644
--- a/runtime/vm/isolate.cc
+++ b/runtime/vm/isolate.cc
@@ -797,6 +797,7 @@ Isolate::Isolate(const Dart_IsolateFlags& api_flags)
       symbols_mutex_(new Mutex()),
       type_canonicalization_mutex_(new Mutex()),
       constant_canonicalization_mutex_(new Mutex()),
+      megamorphic_lookup_mutex_(new Mutex()),
       message_handler_(NULL),
       spawn_state_(NULL),
       is_runnable_(false),
@@ -863,6 +864,8 @@ Isolate::~Isolate() {
   type_canonicalization_mutex_ = NULL;
   delete constant_canonicalization_mutex_;
   constant_canonicalization_mutex_ = NULL;
+  delete megamorphic_lookup_mutex_;
+  megamorphic_lookup_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.