Add @type {Object} annotations to some properties that are possibly null. When a Polymer property type is inferred by the Closure Polymer Pass using the type property field, it will make it non-nullable (e.g. !Object in this case). We need it to be nullable (which is, confusingly, the default in Closure, hence no ! in the type annotation).
responseUrl->responseURL(https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest/responseURL)@type {Object}annotations to some properties that are possiblynull. When a Polymer property type is inferred by the Closure Polymer Pass using thetypeproperty field, it will make it non-nullable (e.g.!Objectin this case). We need it to be nullable (which is, confusingly, the default in Closure, hence no!in the type annotation).