It was needed to avoid collision of identifiers used in fields
initializers with variables declared in the constructor, but it
broke `this` handling.
This commit removes that behavior and instead renames the
colliding variables.
* First implementation sketch and testcase
* Use helper instead of inline IIFE's
* minNodeVersion 6.0.0
* Hoisted visitor for subtraversal and fixed edgest case
* Resolve merge conflicts in helpers
* Remove duplicated helper from messing up Git
