[es6] Implement @@match subclassing.

test/mjsunit/harmony/string-match.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 = {};
+pattern[Symbol.match] = function(string) {
+  return string.length;
+};
+// Check object coercible fails.
+assertThrows(() => String.prototype.match.call(null, pattern),
+             TypeError);
+// Override is called.
+assertEquals(5, "abcde".match(pattern));
+// Non-callable override.
+pattern[Symbol.match] = "dumdidum";
+assertThrows(() => "abcde".match(pattern), TypeError);
+
+assertEquals("[Symbol.match]", RegExp.prototype[Symbol.match].name);