Also check new_target_is_base() bit when comparing two maps for equivalence.

test/mjsunit/regress/regress-crbug-580506.js

Index: test/mjsunit/regress/regress-crbug-580506.js
diff --git a/test/mjsunit/regress/regress-crbug-580506.js b/test/mjsunit/regress/regress-crbug-580506.js
new file mode 100644
index 0000000000000000000000000000000000000000..fb036982cfbea449c85b31c24834d43216282a49
--- /dev/null
+++ b/test/mjsunit/regress/regress-crbug-580506.js
@@ -0,0 +1,22 @@
+// Copyright 2016 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Flags: --allow-natives-syntax
+
+(function() {
+  'use strict';
+  class A extends Function {
+    constructor(...args) {
+      super(...args);
+      this.a = 42;
+    }
+  }
+  var v1 = new A("'use strict';");
+  function f(func) {
+    func.__defineSetter__('a', function() { });
+  }
+  var v2 = new A();
+  f(v2);
+  f(v1);
+})();