Breaking change introduced in version 1.9.0 for Node 4.X

[robocel]

The change made to the file typescript.js in version 1.9.0 contains an ES6 feature that is not supported on older version of Node (such as Node 4.X):
e5659e0#diff-edcbe97b3f08275a033ef78d5ebb1911R51

If this package is intending to use semantic versioning (of course it's any library maintainers right to do any versioning scheme they'd like), then any reliance on ES6 to the exclusion of systems not yet ready to use those features would constitute a breaking change and should have resulted in version 2.0.0 of the library instead of going to 1.9.0.

In this case, I don't think the ES6 feature is vital, so I'm proposing that the feature in question be replaced with an equivalent ES5 version and released as version 1.9.1. If the decision is made to support ES6 syntax going forward, I would propose then re-releasing the original code as version 2.0.0. I'm happy to submit a PR to this effect, if you agree that this proposal is valid.

Thanks!