[es6] Implement @@split subclassing.

test/mjsunit/harmony/string-split.js

+// Copyright 2015 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: --harmony-regexp-subclass
+
+var pattern = {};
+var limit = { value: 3 };
+pattern[Symbol.split] = function(string, limit) {
+  return string.length * limit.value;
+};
+// Check object coercible fails.
+assertThrows(() => String.prototype.split.call(null, pattern, limit),
+             TypeError);
+// Override is called.
+assertEquals(15, "abcde".split(pattern, limit));
+// Non-callable override.
+pattern[Symbol.split] = "dumdidum";
+assertThrows(() => "abcde".split(pattern, limit), TypeError);
+
+assertEquals("[Symbol.split]", RegExp.prototype[Symbol.split].name);