The _new_NamedTuple helper was in a Base-only branch, causing

```

julia> Core.eval(Core.Compiler, quote f(;a=1) = a end)
f (generic function with 1 method)

julia> Core.Compiler.f(;b=2)
ERROR: UndefVarError: `_new_NamedTuple` not defined
Stacktrace:
 [1] macro expansion
   @ Core.Compiler ./namedtuple.jl:0 [inlined]
 [2] structdiff(a::@NamedTuple{b::Int64}, b::Type{NamedTuple{(:a,)}})
   @ Core.Compiler ./namedtuple.jl:421
 [3] top-level scope
   @ REPL[2]:1
```

After this change, we have the expected
```
julia> Core.eval(Core.Compiler, quote f(;a=1) = a end)
f (generic function with 1 method)

julia> Core.Compiler.f(;b=2)
ERROR: MethodError: no method matching f(; b::Int64)

Closest candidates are:
  f(; a) got unsupported keyword argument "b"
   @ Core REPL[13]:1

Stacktrace:
 [1] kwerr(kw::@NamedTuple{b::Int64}, args::Function)
   @ Core.Compiler ./error.jl:165
 [2] top-level scope
   @ REPL[14]:1
```