Pass dirname as extra metadata to preset constructor. (#4834)

* Pass `dirname` as extra metadata to preset constructor.

Sometimes a preset would like to know where it should resolve relative paths from (e.g. https://github.com/tleunen/babel-plugin-module-resolver) and this extra information makes that possible.

* Test for `dirname` passed into preset constructor

This adds a check for `dirname`’s existence and correctness to the
`resolve-addons-relative-to-file` test, and serves as a minimal example
of a path-aware preset.

packages/babel-core/src/transformation/file/options/option-manager.js

@@ -299,7 +299,7 @@ export default class OptionManager {
             (presetLoc || "a preset") + " which does not accept options.");
         }
 
-        if (typeof val === "function") val = val(context, options);
+        if (typeof val === "function") val = val(context, options, { dirname });
 
         if (typeof val !== "object") {
           throw new Error(`Unsupported preset format: ${val}.`);

packages/babel-core/test/fixtures/resolution/resolve-addons-relative-to-file/node_modules/addons/preset.js

@@ -1,5 +1,10 @@
-module.exports = {
-  plugins: [plugin],
+module.exports = function preset (context, options, fileContext) {
+  if (/resolve-addons-relative-to-file$/.test(fileContext.dirname)) {
+    return {
+      plugins: [plugin],
+    };
+  }
+  return {};
 };
 
 function plugin () {