Move selector and arguments descriptor into MegamorphicCache.

runtime/vm/raw_object.h

Index: runtime/vm/raw_object.h
diff --git a/runtime/vm/raw_object.h b/runtime/vm/raw_object.h
index cfac3ac6513f18722a7df7b19c715264873bc861..5b9f658633ce95873c2677499cf3f4472bc35349 100644
--- a/runtime/vm/raw_object.h
+++ b/runtime/vm/raw_object.h
@@ -1421,8 +1421,10 @@ class RawMegamorphicCache : public RawObject {
   }
   RawArray* buckets_;
   RawSmi* mask_;
+  RawString* target_name_;     // Name of target function.
+  RawArray* args_descriptor_;  // Arguments descriptor.
   RawObject** to() {
-    return reinterpret_cast<RawObject**>(&ptr()->mask_);
+    return reinterpret_cast<RawObject**>(&ptr()->args_descriptor_);
   }
 
   int32_t filled_entry_count_;