Use mutable heapnumbers to store doubles in fields.

src/x64/ic-x64.cc

Index: src/x64/ic-x64.cc
diff --git a/src/x64/ic-x64.cc b/src/x64/ic-x64.cc
index 0a9ceaa8650ba3f9565e5b7c0084324ca59569ea..761e05ae466df7ce6709006b63c26925aa59b392 100644
--- a/src/x64/ic-x64.cc
+++ b/src/x64/ic-x64.cc
@@ -1528,6 +1528,26 @@ void KeyedStoreIC::GenerateRuntimeSetProperty(MacroAssembler* masm,
 }
 
 
+void StoreIC::GenerateSlow(MacroAssembler* masm) {
+  // ----------- S t a t e -------------
+  //  -- rax     : value
+  //  -- rcx     : key
+  //  -- rdx     : receiver
+  //  -- rsp[0]  : return address
+  // -----------------------------------
+
+  __ pop(rbx);
+  __ push(rdx);  // receiver
+  __ push(rcx);  // key
+  __ push(rax);  // value
+  __ push(rbx);  // return address
+
+  // Do tail-call to runtime routine.
+  ExternalReference ref(IC_Utility(kKeyedStoreIC_Slow), masm->isolate());
+  __ TailCallExternalReference(ref, 3, 1);
+}
+
+
 void KeyedStoreIC::GenerateSlow(MacroAssembler* masm) {
   // ----------- S t a t e -------------
   //  -- rax     : value