Introduce {FAST,SLOW}_STRING_WRAPPER_ELEMENTS

src/code-stubs.h

Index: src/code-stubs.h
diff --git a/src/code-stubs.h b/src/code-stubs.h
index ae0c1e9172f99e7ca259e2102196ed5d12d0fa5f..68e5592a800c712386dab197e71ef7f040cfd59d 100644
--- a/src/code-stubs.h
+++ b/src/code-stubs.h
@@ -2725,6 +2725,9 @@ class StoreElementStub : public PlatformCodeStub {
   StoreElementStub(Isolate* isolate, ElementsKind elements_kind,
                    KeyedAccessStoreMode mode)
       : PlatformCodeStub(isolate) {
+    // TODO(jkummerow): Rename this stub to StoreSlowElementStub,
+    // drop elements_kind parameter.
+    DCHECK_EQ(DICTIONARY_ELEMENTS, elements_kind);
     minor_key_ = ElementsKindBits::encode(elements_kind) |
                  CommonStoreModeBits::encode(mode);
   }