fix #392, #393

Author: zloirock

modules/es6.reflect.set.js

@@ -20,9 +20,11 @@ function set(target, propertyKey, V /* , receiver */) {
   }
   if (has(ownDesc, 'value')) {
     if (ownDesc.writable === false || !isObject(receiver)) return false;
-    existingDescriptor = gOPD.f(receiver, propertyKey) || createDesc(0);
-    existingDescriptor.value = V;
-    dP.f(receiver, propertyKey, existingDescriptor);
+    if (existingDescriptor = gOPD.f(receiver, propertyKey)) {
+      if (existingDescriptor.get || existingDescriptor.set || existingDescriptor.writable === false) return false;
+      existingDescriptor.value = V;
+      dP.f(receiver, propertyKey, existingDescriptor);
+    } else dP.f(receiver, propertyKey, createDesc(0, V));
     return true;
   }
   return ownDesc.set === undefined ? false : (ownDesc.set.call(receiver, V), true);