[regexp] parse RegExpUnicodeEscapeSequence according to spec.

test/mjsunit/harmony/unicode-regexp-restricted-syntax.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
 
 // test262/data/test/language/literals/regexp/u-dec-esc
 assertThrows("/\\1/u");
 // test262/language/literals/regexp/u-invalid-char-range-a
 assertThrows("/[\\w-a]/u");
@@ skipping 14 matching lines @@
 assertThrows("/[\\c0]/u");
 // test262/built-ins/RegExp/unicode_restricted_incomple_quantifier
 assertThrows("/a{/u");
 assertThrows("/a{1,/u");
 assertThrows("/{/u");
 assertThrows("/}/u");
 // test262/data/test/built-ins/RegExp/unicode_restricted_brackets
 assertThrows("/]/u");
 // test262/built-ins/RegExp/unicode_identity_escape
 /\//u;
+
+// escaped \0 is allowed inside a character class.
+assertEquals(["\0"], /[\0]/u.exec("\0"));
+// unless it is followed by another digit.
+assertThrows("/[\\00]/u");
+assertThrows("/[\\01]/u");
+assertThrows("/[\\09]/u");
+assertEquals(["\u{0}1\u{0}a\u{0}"], /[1\0a]+/u.exec("b\u{0}1\u{0}a\u{0}2"));
+// escaped \- is allowed inside a character class.
+assertEquals(["-"], /[a\-z]/u.exec("12-34"));