Use __proto__ if available.

lib/chai/utils/addChainableMethod.js

@@ -14,7 +14,7 @@ var transferFlags = require('./transferFlags');
  * Module variables
  */
 
-// This method will need to add properties to a function.
+// Without `__proto__` support, this module will need to add properties to a function.
 // However, some Function.prototype methods cannot be overwritten,
 // and there seems no easy cross-platform way to detect them (@see chaijs/chai/issues/69).
 var excludeNames = /^(?:length|name|arguments|caller)$/;
@@ -60,14 +60,20 @@ module.exports = function (ctx, name, method, chainingBehavior) {
           return result === undefined ? this : result;
         };
 
-        // Re-enumerate every time to better accomodate plugins.
-        var asserterNames = Object.getOwnPropertyNames(ctx);
-        asserterNames.forEach(function (asserterName) {
-          if (!excludeNames.test(asserterName)) {
-            var pd = Object.getOwnPropertyDescriptor(ctx, asserterName);
-            Object.defineProperty(assert, asserterName, pd);
-          }
-        });
+        // Use `__proto__` if available
+        if ('__proto__' in assert) {
+          assert.__proto__ = this;
+        }
+        // Otherwise, redefine all properties (slow!)
+        else {
+          var asserterNames = Object.getOwnPropertyNames(ctx);
+          asserterNames.forEach(function (asserterName) {
+            if (!excludeNames.test(asserterName)) {
+              var pd = Object.getOwnPropertyDescriptor(ctx, asserterName);
+              Object.defineProperty(assert, asserterName, pd);
+            }
+          });
+        }
 
         transferFlags(this, assert);
         return assert;