Add ES2015 RegExp full subclassing semantics behind a flag

src/messages.h

Index: src/messages.h
diff --git a/src/messages.h b/src/messages.h
index 300030832a2a00cca632c85a1d033eb87131aa76..b54161b65776529f1ec13c9bda176c3aed2e09aa 100644
--- a/src/messages.h
+++ b/src/messages.h
@@ -136,6 +136,8 @@ class CallSite {
     "Function has non-object prototype '%' in instanceof check")               \
   T(InvalidArgument, "invalid_argument")                                       \
   T(InvalidInOperatorUse, "Cannot use 'in' operator to search for '%' in %")   \
+  T(InvalidRegExpExecResult,                                                   \
+    "RegExp exec method returned something other than an Object or null")      \
   T(InvalidSimdOperation, "% is not a valid type for this SIMD operation.")    \
   T(IteratorResultNotAnObject, "Iterator result % is not an object")           \
   T(IteratorValueNotAnObject, "Iterator value % is not an entry object")       \