See 2489e2ab for reasoning

With ExtJS 4, we introduced an override to Ext.grid.feature.Chunking
to fix scrollings problems in the grid when having a high number
of nodes. Ext.grid.feature.Chunking has been removed from ExtJS
in version 5, so we hope either the problem is fixed on ExtJS side,
or we will have to find a different workaround.

Ext.ModelMgr.create() was removed in 5.*
The recommended way to create a new instance is to call Ext.create()
Ext.create() does not take a "id" argument, however it sets
the id of the instance to the id  of the config object given
to the Ext.create() call.

This has been renamed to 'rootProperty:' in ExtJS 5. Rename our parameter
accordingly to silence the deprecation warning.

and default values from constructor to class body, needed fix for

"Cannot override method afterRequest on Ext.data.proxy.Ajax instance"

error. See:

https://fiddle.sencha.com/#fiddle/lpe for test fiddle
http://bit.ly/1HJJ0Ej for background

It is no longer possible to download the key without that.