[regexp] support \p in character classes.

test/mjsunit/harmony/regexp-property-disabled.js

+// 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: --harmony-unicode-regexps --no-harmony-regexp-property
+
+function test(source, message) {
+  try {
+    eval(source);
+  } catch (e) {
+    assertEquals(message, e.message);
+    return;
+  }
+  assertUnreachable();
+}
+
+test("/\\pL/u", "Invalid regular expression: /\\pL/: Invalid escape");
+test("/[\\p{L}]/u", "Invalid regular expression: /[\\p{L}]/: Invalid escape");