Performance of getSemanticDiagnostics in compiler API

The setup is somewhat similar to the one in the compiler API example (https://github.com/Microsoft/TypeScript/wiki/Using-the-Compiler-API#incremental-build-support-using-the-language-services) The problem, as in the example, is that errors are reported only for files that were directly changed. It doesn't cover the case in which the changed file was a dependency of another file, and even though the file depending on it was not changed, it will still have errors, which won't be reported. To be more specific, the setup I am talking about is ts-loader + webpack. What happens is that `getSemanticDiagnostics(file)` has to be called for every file in the project, which ends up increasing the incremental build times considerably. Even `program.getSemanticDiagnostics()` iterates through all of the source files as it is seen in the typescript source code. I can only see this problem fixed from the compiler API. It could expose a method which finds all files dependent on a given file. Because in an incremental build only a single file changed, this should be faster than calling `getSemanticDiagnostics()` for each and every project file. The alternative would be some sort of caching on `getSemanticDiagnostics()` itself, but the compiler would probably have to do internally the same work as above to invalidate the cache on file change. This seems to be equivalent to a `getSemanticDiagnosticsForFileAndDependents(file)`-like function.