_createMacroExpression closes over code that other transforms may edit.

Take for instance the following

```ts
assert(`my assertion`, SOME_VAR);
```

This library will close over `SOME_VAR` when building out the transformation ` _createMacroExpression`. Since the built transformations are not flushed until program exit, other transforms that edit or replace `SOME_VAR` will have no affect.