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The setEditable method is only valid for feature layers created from a feature collection not for layers in a feature service.
http://help.arcgis.com/en/webapi/javascript/arcgis/jsapi/featurelayer.html#FeatureLayer/setEditable
If you use this method on a feature service layer you should see the following message in the developer console:
FeatureLayer:setEditable - this functionality is not yet supported for layer in a feature service