[regexp] parse RegExpUnicodeEscapeSequence according to spec.

test/mjsunit/harmony/unicode-regexp-restricted-syntax.js

Index: test/mjsunit/harmony/unicode-regexp-restricted-syntax.js
diff --git a/test/mjsunit/harmony/unicode-regexp-restricted-syntax.js b/test/mjsunit/harmony/unicode-regexp-restricted-syntax.js
index 8c2de8dfa6f0483d4f33c9fddf855238898e508e..76d0539e48160fec7a16a296f5364832c5e5646c 100644
--- a/test/mjsunit/harmony/unicode-regexp-restricted-syntax.js
+++ b/test/mjsunit/harmony/unicode-regexp-restricted-syntax.js
@@ -32,3 +32,13 @@ assertThrows("/}/u");
 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"));