Convert String to a class.

runtime/vm/object_store.h

Index: runtime/vm/object_store.h
diff --git a/runtime/vm/object_store.h b/runtime/vm/object_store.h
index 301293e6758d6821b1285a6c1744cd2afc2486a3..82dff54e1ce569847a262a2ab6307a86c3d816ea 100644
--- a/runtime/vm/object_store.h
+++ b/runtime/vm/object_store.h
@@ -102,9 +102,9 @@ class ObjectStore {
   RawClass* bigint_class() const { return bigint_class_; }
   void set_bigint_class(const Class& value) { bigint_class_ = value.raw(); }
 
-  RawType* string_interface() const { return string_interface_; }
-  void set_string_interface(const Type& value) {
-    string_interface_ = value.raw();
+  RawType* string_type() const { return string_type_; }
+  void set_string_type(const Type& value) {
+    string_type_ = value.raw();
   }
 
   RawClass* one_byte_string_class() const { return one_byte_string_class_; }
@@ -484,7 +484,7 @@ class ObjectStore {
   RawClass* bigint_class_;
   RawClass* double_class_;
   RawType* double_type_;
-  RawType* string_interface_;
+  RawType* string_type_;
   RawClass* one_byte_string_class_;
   RawClass* two_byte_string_class_;
   RawClass* four_byte_string_class_;