`acorn-walk` does not work with injected plugins

Hello,

I've been attempting to use acorn-walk with the acorn-dynamic-import plugin both before and after the version 6 update. So far, have been quite unsuccessful with even the most basic examples.

I created a repo demonstrating the issue: https://github.com/kristoferbaxter/dynamic-walk

Essentially it appears that extensions to base are not leveraged within the simple or ancestor methods.

https://github.com/ampproject/rollup-plugin-closure-compiler/issues/48 was the start of this issue – trying to add support for dynamic import to a rollup plugin for minification with closure compiler.