Beware of Greeks bearing gifts, the old saying goes — or, in today’s case, of Massachusetts Democrats offering cooperation. That’s the warning to President Trump in light of Senator Warren’s vow that she’s “ready to work with” Republicans on “policies that rebuild the middle class.” The GOP would be wise to view offers like Ms. Warren’s with caution, though, despite the temptation among neo-populists to embrace regulation and subsidies as policy tools.