Skip calling has_property() before get_property()

Instead just do the normal Javascript thing of checking if we got `undefined`.
This nearly doubles the speed of value_for_property() as nearly all of the time
was spent in looking up the properties in the JS object.

src/js_object_accessor.hpp

@@ -68,10 +68,10 @@ public:
 
     OptionalValue value_for_property(ValueType dict, std::string const& prop_name, size_t prop_index) {
         ObjectType object = Value::validated_to_object(m_ctx, dict);
-        if (!Object::has_property(m_ctx, object, prop_name)) {
+        ValueType value = Object::get_property(m_ctx, object, prop_name);
+        if (Value::is_undefined(m_ctx, value)) {
             return util::none;
         }
-        ValueType value = Object::get_property(m_ctx, object, prop_name);
         const auto& prop = m_object_schema->persisted_properties[prop_index];
         if (!Value::is_valid_for_property(m_ctx, value, prop)) {
             throw TypeErrorException(*this, m_object_schema->name, prop, value);

src/js_types.hpp

@@ -210,8 +210,6 @@ struct Object {
     static ValueType get_prototype(ContextType, const ObjectType &);
     static void set_prototype(ContextType, const ObjectType &, const ValueType &);
 
-    static bool has_property(ContextType, const ObjectType &, const String<T> &);
-    static bool has_property(ContextType, const ObjectType &, uint32_t);
     static ValueType get_property(ContextType, const ObjectType &, const String<T> &);
     static ValueType get_property(ContextType, const ObjectType &, uint32_t);
     static void set_property(ContextType, const ObjectType &, const String<T> &, const ValueType &, PropertyAttributes attributes = None);
@@ -223,10 +221,11 @@ struct Object {
 
     template<typename P>
     static ValueType validated_get_property(ContextType ctx, const ObjectType &object, const P &property, const char *message = nullptr) {
-        if (!has_property(ctx, object, property)) {
+        auto value = get_property(ctx, object, property);
+        if (Value<T>::is_undefined(ctx, value)) {
             throw std::out_of_range(message ? message : "Object missing expected property: " + util::to_string(property));
         }
-        return get_property(ctx, object, property);
+        return value;
     }
 
     static uint32_t validated_get_length(ContextType ctx, const ObjectType &object) {

src/jsc/jsc_object.hpp

@@ -23,16 +23,6 @@
 namespace realm {
 namespace js {
 
-template<>
-inline bool jsc::Object::has_property(JSContextRef ctx, const JSObjectRef &object, const jsc::String &key) {
-    return JSObjectHasProperty(ctx, object, key);
-}
-
-template<>
-inline bool jsc::Object::has_property(JSContextRef ctx, const JSObjectRef &object, uint32_t index) {
-    return JSObjectHasProperty(ctx, object, jsc::String(util::to_string(index)));
-}
-
 template<>
 inline JSValueRef jsc::Object::get_property(JSContextRef ctx, const JSObjectRef &object, const jsc::String &key) {
     JSValueRef exception = nullptr;

src/node/node_object.hpp

@@ -23,16 +23,6 @@
 namespace realm {
 namespace js {
 
-template<>
-inline bool node::Object::has_property(v8::Isolate* isolate, const v8::Local<v8::Object> &object, const node::String &key) {
-    return Nan::Has(object, key).FromMaybe(false);
-}
-
-template<>
-inline bool node::Object::has_property(v8::Isolate* isolate, const v8::Local<v8::Object> &object, uint32_t index) {
-    return Nan::Has(object, index).FromMaybe(false);
-}
-
 template<>
 inline v8::Local<v8::Value> node::Object::get_property(v8::Isolate* isolate, const v8::Local<v8::Object> &object, const node::String &key) {
     Nan::TryCatch trycatch;