Add support for fs.realpath.native in envs that support it (#682)

* Add support for fs.realpath.native in envs that support it

* Add test for realpathSync.native

* assert.equal -> assert.strictEqual

lib/fs/__tests__/realpath.test.js

@@ -0,0 +1,33 @@
+'use strict'
+
+const fs = require('fs')
+const fse = require('../..')
+const assert = require('assert')
+
+/* eslint-env mocha */
+
+// fs.realpath.native only available in Node v9.2+
+if (typeof fs.realpath.native === 'function') {
+  describe('realpath.native', () => {
+    it('works with callbacks', () => {
+      fse.realpath.native(__dirname, (err, path) => {
+        assert.ifError(err)
+        assert.strictEqual(path, __dirname)
+      })
+    })
+
+    it('works with promises', (done) => {
+      fse.realpath.native(__dirname)
+        .then(path => {
+          assert.strictEqual(path, __dirname)
+          done()
+        })
+        .catch(done)
+    })
+
+    it('works with sync version', () => {
+      const path = fse.realpathSync.native(__dirname)
+      assert.strictEqual(path, __dirname)
+    })
+  })
+}

lib/fs/index.js

@@ -102,3 +102,8 @@ exports.write = function (fd, buffer, ...args) {
     })
   })
 }
+
+// fs.realpath.native only available in Node v9.2+
+if (typeof fs.realpath.native === 'function') {
+  exports.realpath.native = u(fs.realpath.native)
+}