[es6] Reenable new.target + Reflect.construct tests

BUG=v8:3887
LOG=N
R=caitpotter88@gmail.com, adamk@chromium.org

Review URL: https://codereview.chromium.org/1215973005

Cr-Commit-Position: refs/heads/master@{#29359}

test/mjsunit/harmony/new-target.js

@@ -156,8 +156,6 @@
 })();
 
 
-/*
-// TODO(arv): Reflect.construct does not work correctly with a third argument.
 (function TestReflectConstruct() {
   var calls = 0;
   function f(expected) {
@@ -187,7 +185,6 @@
   assertEquals(Object.getPrototypeOf(o), g.prototype);
   assertEquals(2, calls);
 })();
-*/
 
 
 (function TestRestParametersFunction() {