exportSpecifier builder and probably others are broken in ^0.14.0

I don't fully understand what's supposed to happen when a node type is defined in two places (esprima and es6) but it seems to be causing problems for `exportSpecifier`:

```
> var t = require('ast-types').builders
undefined
> t.exportSpecifier()
Uncaught:
Error: no value or default function given for field "exported" of ExportSpecifier("id": Identifier | null, "name": Identifier | null)
at addParam (/Users/andy/temp/node_modules/ast-types/lib/types.js:443:27)
at /Users/andy/temp/node_modules/ast-types/lib/types.js:477:21
at Array.forEach ()
at Object.builder [as exportSpecifier] (/Users/andy/temp/node_modules/ast-types/lib/types.js:475:46)
> t.exportSpecifier(t.identifier('foo'), t.identifier('bar'))
Uncaught:
Error: no value or default function given for field "exported" of ExportSpecifier("id": Identifier | null, "name": Identifier | null)
at addParam (/Users/andy/temp/node_modules/ast-types/lib/types.js:443:27)
at /Users/andy/temp/node_modules/ast-types/lib/types.js:477:21
at Array.forEach ()
at Object.builder [as exportSpecifier] (/Users/andy/temp/node_modules/ast-types/lib/types.js:475:46)
```

Can't reproduce this issue in 0.13.4.

I noticed that field names `id` and `name` come from the esprima defs, whereas the field name `exported` comes from the es6 defs.

Reproduce the two exportSpecifier builder calls and start in lib/types.js at the stack-traced builder logic. Compare the esprima and es6 definitions for ExportSpecifier and determine how their fields are combined; done means both calls work without the missing-field error while preserving the behavior from 0.13.4.

Written by the indexing model from the issue text.