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 dac623fa8f3199c5fad858775e5b5bced0fcd50a..ca467ea46a3cf5d50f2e4d1796b83d1c65027c5a 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")       \