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

runtime/vm/isolate.h

Index: runtime/vm/isolate.h
diff --git a/runtime/vm/isolate.h b/runtime/vm/isolate.h
index 701579cd8793aa95f34f485038fc9ac3529f791a..4e6e1127aab9c803918c60d87bc032245bd10912 100644
--- a/runtime/vm/isolate.h
+++ b/runtime/vm/isolate.h
@@ -282,6 +282,9 @@ class Isolate : public BaseIsolate {
   Mutex* constant_canonicalization_mutex() const {
     return constant_canonicalization_mutex_;
   }
+  Mutex* megamorphic_lookup_mutex() const {
+    return megamorphic_lookup_mutex_;
+  }
 
   Debugger* debugger() const {
     if (!FLAG_support_debugger) {
@@ -717,6 +720,7 @@ class Isolate : public BaseIsolate {
   Mutex* symbols_mutex_;  // Protects concurrent access to the symbol table.
   Mutex* type_canonicalization_mutex_;  // Protects type canonicalization.
   Mutex* constant_canonicalization_mutex_;  // Protects const canonicalization.
+  Mutex* megamorphic_lookup_mutex_;  // Protects megamorphic table lookup.
   MessageHandler* message_handler_;
   IsolateSpawnState* spawn_state_;
   bool is_runnable_;