chore(http): add XMLHttpRequest as type on XHRConnection

Pending merging of https://github.com/angular/ts2dart/pull/232
`XHRConnection` private var `_xhr` should be typed as such: https://github.com/angular/angular/blob/b3d98cba771c2bd57c2f061f990be16417037f55/modules/angular2/src/http/backends/xhr_backend.ts#L27
