Revert "defaultExtension forwards compatibility"
Revert "defaultExtension forwards compatibility"

This reverts commit 9cb5f3a28fe43601489a13c9021fb2a2b8f0cfdf.

systemjs/builder,alexisvincent/builder,jrauschenbusch/builder,systemjs/builder,jrauschenbusch/builder,alexisvincent/builder,alexisvincent/builder,systemjs/builder,jrauschenbusch/builder