AK: JsonParser improvements

- Parsing invalid JSON no longer asserts
    Instead of asserting when coming across malformed JSON,
    JsonParser::parse now returns an Optional<JsonValue>.
- Disallow trailing commas in JSON objects and arrays
- No longer parse 'undefined', as that is a purely JS thing
- No longer allow non-whitespace after anything consumed by the initial
  parse() call. Examples of things that were valid and no longer are:
    - undefineddfz
    - {"foo": 1}abcd
    - [1,2,3]4
- JsonObject.for_each_member now iterates in original insertion order

AK/JsonObject.h

@@ -41,26 +41,32 @@ public:
     ~JsonObject() {}
 
     JsonObject(const JsonObject& other)
-        : m_members(other.m_members)
+        : m_order(other.m_order)
+        , m_members(other.m_members)
     {
     }
 
     JsonObject(JsonObject&& other)
-        : m_members(move(other.m_members))
+        : m_order(move(other.m_order))
+        , m_members(move(other.m_members))
     {
     }
 
     JsonObject& operator=(const JsonObject& other)
     {
-        if (this != &other)
+        if (this != &other) {
             m_members = other.m_members;
+            m_order = other.m_order;
+        }
         return *this;
     }
 
     JsonObject& operator=(JsonObject&& other)
     {
-        if (this != &other)
+        if (this != &other) {
             m_members = move(other.m_members);
+            m_order = move(other.m_order);
+        }
         return *this;
     }
 
@@ -70,7 +76,7 @@ public:
     JsonValue get(const String& key) const
     {
         auto* value = get_ptr(key);
-        return value ? *value : JsonValue(JsonValue::Type::Undefined);
+        return value ? *value : JsonValue(JsonValue::Type::Null);
     }
 
     JsonValue get_or(const String& key, JsonValue alternative) const
@@ -94,14 +100,17 @@ public:
 
     void set(const String& key, JsonValue value)
     {
+        m_order.append(key);
         m_members.set(key, move(value));
     }
 
     template<typename Callback>
     void for_each_member(Callback callback) const
     {
-        for (auto& it : m_members)
-            callback(it.key, it.value);
+        for (size_t i = 0; i < m_order.size(); ++i) {
+            auto property = m_order[i];
+            callback(property, m_members.get(property).value());
+        }
     }
 
     template<typename Builder>
@@ -113,6 +122,7 @@ public:
     String to_string() const { return serialized<StringBuilder>(); }
 
 private:
+    Vector<String> m_order;
     HashMap<String, JsonValue> m_members;
 };
 
@@ -193,9 +203,6 @@ inline void JsonValue::serialize(Builder& builder) const
     case Type::UnsignedInt64:
         builder.appendf("%llu", as_u64());
         break;
-    case Type::Undefined:
-        builder.append("undefined");
-        break;
     case Type::Null:
         builder.append("null");
         break;