Make assert.AssertionError instance of Error

lib/assert.js

@@ -47,6 +47,7 @@ assert.AssertionError = function AssertionError (options) {
     Error.captureStackTrace(this, stackStartFunction);
   }
 };
+process.inherits(assert.AssertionError, Error);
 
 assert.AssertionError.prototype.toString = function() {
   if (this.message) {