fix(ngMock.$interval): should use angular.isDefined

The newly introduced `$interval` mock service for ngMock calls `isDefined`
in the global namespace which fails when used within unit tests.

This change adds the missing `angular.` prefix to such `isDefined` calls.

Closes #4334
Closes #4353
Merge branch 'master' of https://github.com/angular/angular.js
componentizd!