Stop using expensive directoryExists in unit/rwc tests where it does not matter

[weswigham]

The harness directoryExists implementation iterated over every file in the compilation to see if any started with the given path in order to return true. directoryExists is only used as a "shortcut" in module resolution to avoid doing exists calls on a bunch of files within a directory. In the test harness, unlike on disk, the directory exists check is crazy expensive for large tests (like RWC tests - this was ~20s of time on our largest test!), while the file stat is almost free.

With this change, the harness directoryExists implementation becomes () => true for all tests where the difference is not observable - the old implementation is now only used by tests which baseline module resolution traces.

[mihailik]

Note that directoryExists is declared as optional in types.ts ModuleResoultionHost (which CompilerHost extends):

https://github.com/Microsoft/TypeScript/blob/a6555921f4b1d51281b1241c69e91e5b82d2f07e/src/compiler/types.ts#L4012

I would have thought cases like that is exactly where null would make sense?

[mihailik]

Oh it seems to be assumed non-null in many cases.

Maybe the declaration shouldn't be optional then?

https://github.com/Microsoft/TypeScript/blob/d05443bb1d3c346fbef4c2468ff06b00041b688d/src/services/jsTyping.ts#L205
https://github.com/Microsoft/TypeScript/blob/8fc651870e44efd12598a42ae84fb0e19067fcfd/src/compiler/watch.ts#L187

https://github.com/Microsoft/TypeScript/blob/8a292e35147e287cb18057689923295d79391dea/src/compiler/core.ts#L2837