Revert "Revert ast-types/fork changes to reinstate JSX types."

This reverts commit 0598995.

When we no longer use ast-types, this won't matter, but that's waiting on
a relatively significant PR (standard-things#87).

lib/compiler.js

@@ -1,6 +1,10 @@
 var hasOwn = Object.prototype.hasOwnProperty;
 var assert = require("assert");
-var types = require("ast-types");
+var types = require("ast-types/fork")([
+  require("ast-types/def/esprima"),
+  require("ast-types/def/babel6-core")
+]);
+
 var n = types.namedTypes;
 var b = types.builders;
 var MagicString = require("magic-string/dist/magic-string.cjs.js");