`b.path` (`buildPath`/`buildHead`) does not work with `@namedArg`

[chancancode]

b.path('@namedArg') should return something like:

{
  type: "PathExpression",
  original: "@namedArg",
  this: false,
  parts: ["namedArg"],
  data: true
}

but instead returns something like:

{
  type: "PathExpression",
  original: "@namedArg",
  this: false,
  parts: ["@namedArg"],
  data: false
}

At runtime, this causes it to do an implicit this lookup (this["@namedArg"]) instead.

[rwjblue]

Ya, I ran into this somewhere as well but never tracked it down. We should fix...