[AST] Add method to concatenate AstRawStrings

src/messages.h

Index: src/messages.h
diff --git a/src/messages.h b/src/messages.h
index b3f8a1d5e154191ce141fd9a4e06f037679fa70d..8f0a8ccb33171e0d75a0b16e86bd042243d1e81c 100644
--- a/src/messages.h
+++ b/src/messages.h
@@ -288,6 +288,7 @@ class ErrorUtils : public AllStatic {
   T(MethodInvokedOnNullOrUndefined,                                            \
     "Method invoked on undefined or null value.")                              \
   T(MethodInvokedOnWrongType, "Method invoked on an object that is not %.")    \
+  T(MissingPrivateField, "Object does not have this private field.")           \
   T(NoAccess, "no access")                                                     \
   T(NonCallableInInstanceOfCheck,                                              \
     "Right-hand side of 'instanceof' is not callable")                         \