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

runtime/vm/megamorphic_cache_table.cc

Index: runtime/vm/megamorphic_cache_table.cc
diff --git a/runtime/vm/megamorphic_cache_table.cc b/runtime/vm/megamorphic_cache_table.cc
index 4705b21900c3c4396ba436edcf486ab8fe5e9aaa..e3d34dd01c5ad7b318116172c60520c466663137 100644
--- a/runtime/vm/megamorphic_cache_table.cc
+++ b/runtime/vm/megamorphic_cache_table.cc
@@ -16,7 +16,7 @@ RawMegamorphicCache* MegamorphicCacheTable::Lookup(Isolate* isolate,
                                                    const String& name,
                                                    const Array& descriptor) {
   // Multiple compilation threads could access this lookup.
-  SafepointMutexLocker ml(isolate->mutex());
+  SafepointMutexLocker ml(isolate->megamorphic_lookup_mutex());
   ASSERT(name.IsSymbol());
   // TODO(rmacnak): ASSERT(descriptor.IsCanonical());