Add test for early error in RegExp literals (#723)

test/language/literals/regexp/early-err-flags-unicode-escape.js

@@ -0,0 +1,13 @@
+// Copyright (C) 2016 the V8 project authors. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+/*---
+esid: sec-literals-regular-expression-literals-static-semantics-early-errors
+es6id: 11.8.5.1
+description: >
+  RegularExpressionFlags :: RegularExpressionFlags IdentifierPart
+
+  - It is a Syntax Error if IdentifierPart contains a Unicode escape sequence.
+negative: SyntaxError
+---*/
+
+/./\u0067;