Merge pull request #490 from astorije/patch-2

Fix wrong call to the underlying method when checking if an object is frozen

lib/chai/core/assertions.js

@@ -1720,7 +1720,7 @@ module.exports = function (chai, _) {
     var obj = flag(this, 'object');
 
     this.assert(
-      Object.isSealed(obj)
+      Object.isFrozen(obj)
       , 'expected #{this} to be frozen'
       , 'expected #{this} to not be frozen'
     );