Replace HeapNumber as doublebox with an explicit MutableHeapNumber.

src/hydrogen.cc

Index: src/hydrogen.cc
diff --git a/src/hydrogen.cc b/src/hydrogen.cc
index ec5bb9bc8f91af24598efcb31b0242492af5be5a..da20c025e1a7d746977f9f5bc58894a1f472b144 100644
--- a/src/hydrogen.cc
+++ b/src/hydrogen.cc
@@ -5781,8 +5781,9 @@ HInstruction* HOptimizedGraphBuilder::BuildStoreNamedField(
       HInstruction* heap_number = Add<HAllocate>(heap_number_size,
           HType::HeapObject(),
           NOT_TENURED,
-          HEAP_NUMBER_TYPE);
-      AddStoreMapConstant(heap_number, isolate()->factory()->heap_number_map());
+          MUTABLE_HEAP_NUMBER_TYPE);
+      AddStoreMapConstant(
+          heap_number, isolate()->factory()->mutable_heap_number_map());
       Add<HStoreNamedField>(heap_number, HObjectAccess::ForHeapNumberValue(),
                             value);
       instr = New<HStoreNamedField>(checked_object->ActualValue(),
@@ -10953,11 +10954,14 @@ void HOptimizedGraphBuilder::BuildEmitInObjectProperties(
         // 2) we can just use the mode of the parent object for pretenuring
         HInstruction* double_box =
             Add<HAllocate>(heap_number_constant, HType::HeapObject(),
-                pretenure_flag, HEAP_NUMBER_TYPE);
+                pretenure_flag, MUTABLE_HEAP_NUMBER_TYPE);
         AddStoreMapConstant(double_box,
-            isolate()->factory()->heap_number_map());
-        Add<HStoreNamedField>(double_box, HObjectAccess::ForHeapNumberValue(),
-                              Add<HConstant>(value));
+            isolate()->factory()->mutable_heap_number_map());
+        // Unwrap the mutable heap number from the boilerplate.
+        HValue* double_value =
+            Add<HConstant>(Handle<HeapNumber>::cast(value)->value());
+        Add<HStoreNamedField>(
+            double_box, HObjectAccess::ForHeapNumberValue(), double_value);
         value_instruction = double_box;
       } else if (representation.IsSmi()) {
         value_instruction = value->IsUninitialized()