Do not use non-standard `String#contains` which may not be available.

In the tests the traceur runtime (included with es6-module-transpiler < v0.4.x) polyfills `String#contains`. When not including anything polyfilling this it will not be available.
Merge pull request #1999 from eventualbuddha/remove-nonstandard-string-contains

Do not use non-standard `String#contains` which may not be available.