Errors with Typescript 2.6 using --strictFunctionTypes

I should note that this is using version `1.11.9`:

```sh
$ tsc --strictFunctionTypes
dojo/1.11/_base.d.ts(1853,4): error TS2411: Property ''text'' of type '(xhr: { responseText: string; }) => string' is not assignable to string index type '(xhr: { responseText?: string; responseXML?: string; }) => any'.
dojo/1.11/_base.d.ts(1854,4): error TS2411: Property ''json'' of type '(xhr: { responseText: string; }) => GenericObject' is not assignable to string index type '(xhr: { responseText?: string; responseXML?: string; }) => any'.
dojo/1.11/_base.d.ts(1855,4): error TS2411: Property ''json-comment-filtered'' of type '(xhr: { responseText: string; }) => GenericObject' is not assignable to string index type '(xhr: { responseText?: string; responseXML?: string; }) => any'.
dojo/1.11/_base.d.ts(1856,4): error TS2411: Property ''javascript'' of type '(xhr: { responseText: string; }) => any' is not assignable to string index type '(xhr: { responseText?: string; responseXML?: string; }) => any'.
dojo/1.11/_base.d.ts(1857,4): error TS2411: Property ''xml'' of type '(xhr: { responseXML: string; }) => Document' is not assignable to string index type '(xhr: { responseText?: string; responseXML?: string; }) => any'.
dojo/1.11/_base.d.ts(1858,4): error TS2411: Property ''json-comment-optional'' of type '(xhr: { responseText: string; }) => GenericObject' is not assignable to string index type '(xhr: { responseText?: string; responseXML?: string; }) => any'.
```