Project not recompiling when definition files are changed

[liangchunn]

When importing external interfaces or types, the compiler will not re-compile even though the definition files have changed.

This causes issues where importing definitions during incremental builds lead to failures. This is because type definition files do not emit any JS files, and webpack will not watch them.

An example repo and steps to reproduce can be found here:
https://github.com/liangchunn/not-recompiling-ts

TypeStrong/ts-loader#697