feat(vanilla): add testing infrastructure and tests (NK-wjnczv)

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+# estree-walker
+
+Simple utility for walking an [ESTree](https://github.com/estree/estree)-compliant AST, such as one generated by [acorn](https://github.com/marijnh/acorn).
+
+
+## Installation
+
+```bash
+npm i estree-walker
+```
+
+
+## Usage
+
+```js
+var walk = require('estree-walker').walk;
+var acorn = require('acorn');
+
+ast = acorn.parse(sourceCode, options); // https://github.com/acornjs/acorn
+
+walk(ast, {
+  enter(node, parent, prop, index) {
+    // some code happens
+  },
+  leave(node, parent, prop, index) {
+    // some code happens
+  }
+});
+```
+
+Inside the `enter` function, calling `this.skip()` will prevent the node's children being walked, or the `leave` function (which is optional) being called.
+
+Call `this.replace(new_node)` in either `enter` or `leave` to replace the current node with a new one.
+
+Call `this.remove()` in either `enter` or `leave` to remove the current node.
+
+## Why not use estraverse?
+
+The ESTree spec is evolving to accommodate ES6/7. I've had a couple of experiences where [estraverse](https://github.com/estools/estraverse) was unable to handle an AST generated by recent versions of acorn, because it hard-codes visitor keys.
+
+estree-walker, by contrast, simply enumerates a node's properties to find child nodes (and child lists of nodes), and is therefore resistant to spec changes. It's also much smaller. (The performance, if you're wondering, is basically identical.)
+
+None of which should be taken as criticism of estraverse, which has more features and has been battle-tested in many more situations, and for which I'm very grateful.
+
+
+## License
+
+MIT