bug: If property doesn't exist on object, don't throw KeyNotFoundExce…

…ption

The spec says to check if the descriptor is undefined when calling
OrdinaryGet. Before this commit, we always tried to get the property
from the internal dictionary.

This would cause internal errors, throwing a KeyNotFoundException if the
property did not exist on the object.

JSS.Lib/AST/Values/Object.cs

@@ -172,11 +172,18 @@ internal Completion Get(string P, Object receiver)
     // 10.1.8.1 OrdinaryGet ( O, P, Receiver ), https://tc39.es/ecma262/#sec-ordinaryget
     static internal Completion OrdinaryGet(Object O, string P, Object receiver)
     {
-        // FIXME: 1. Let desc be ? O.[[GetOwnProperty]](P).
-        // FIXME: 2. If desc is undefined, then
-        // FIXME: a. Let parent be ? O.[[GetPrototypeOf]]().
-        // FIXME: b. If parent is null, return undefined.
-        // FIXME: c. Return ? parent.[[Get]](P, Receiver).
+        // 1. Let desc be ? O.[[GetOwnProperty]](P).
+        var desc = O.GetOwnProperty(P);
+        if (desc.IsAbruptCompletion()) return desc;
+
+        // 2. If desc is undefined, then
+        if (desc.Value.IsUndefined())
+        {
+            // FIXME: a. Let parent be ? O.[[GetPrototypeOf]]().
+            // FIXME: b. If parent is null, return undefined.
+            // FIXME: c. Return ? parent.[[Get]](P, Receiver).
+            return Undefined.The;
+        }
 
         // FIXME: 3. If IsDataDescriptor(desc) is true, return desc.[[Value]].
         return O.DataProperties[P].Value;