Allow the ZWJ and ZWNJ characters in IdentifierPart as per ES5.

src/char-predicates.h

Index: src/char-predicates.h
===================================================================
--- src/char-predicates.h	(revision 10785)
+++ src/char-predicates.h	(working copy)
@@ -57,6 +57,8 @@
   static inline bool Is(uc32 c) {
     return IdentifierStart::Is(c)
         || unibrow::Number::Is(c)
+        || c == 0x200C  // U+200C is Zero-Width Non-Joiner.
+        || c == 0x200D  // U+200D is Zero-Width Joiner.
         || unibrow::CombiningMark::Is(c)
         || unibrow::ConnectorPunctuation::Is(c);
   }