This undoes the property call folding from #6656.
It complicates the private property transforms, since they boil down to `map.set(this, vlaue)` and we definitely don't want the next call define a property on the map.
/cc @Andarist
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.
