This node is virtual AST node, I think it sometimes confuse users.
So introducing `Variable.ImplicitGlobal` type and using it is better.

For example,
```js
if (/* global variable check */) {
  globalScope.__define(node.left, {
    type: Variable.ImplcitGlobal,
    name: node.left,
    node: node
  });
}
currentScope.__referencing(node.left, Reference.WRITE, node.right);
currentScope.__referencing(node.right);
```