Fix service host interface to support ts2.5

[cevek]

In typescript 2.5 language service host needs to implement fileExists, readFile and readDirectory to be able to correctly resolve the modules (otherwise only ambient modules would be resolved)
Described here https://github.com/Microsoft/TypeScript/blame/master/src/services/types.ts#L166

[coveralls]

Coverage remained the same at 86.538% when pulling 09c7482 on cevek:fix-ts2.5 into 28697b1 on TypeStrong:master.

[blakeembrey]

We have existing implementations of two of these functions, should we just use those?

[cevek]

@blakeembrey Fixed

[coveralls]

Coverage remained the same at 86.538% when pulling a99486f on cevek:fix-ts2.5 into 28697b1 on TypeStrong:master.

[johnnyreilly]

@blakeembrey - do you know if we need to make a similar change to ts-loader here: https://github.com/TypeStrong/ts-loader/blob/master/src/servicesHost.ts#L34 ?

[blakeembrey]

@johnnyreilly I would think so, yes. You can just use the ts.sys functions though.

[johnnyreilly]

Thanks @blakeembrey - I've tested and it looks good.