fix: clean up system temp directory when running.

While the system temp directory gets cleaned up semi-regularly,
on long-running machines (e.g. CI servers), it is possible for
the PID to get reused before the system temp directory gets
cleared. When this happens, the temp directory we create as an
intermediate location for TypeScript build artifacts (to prevent
churn in the build) can end up being used for multiple builds,
even across separate apps. This can result in very bad output
from the build, including total runtime failures from things like
`require` entries including files from different builds or apps.

Prevent this by always running `fs.removeSync` on the temp output
directory prior to setting it and putting new output there.

Fixes #318.

ts/lib/incremental-typescript-compiler/index.js

@@ -117,6 +117,8 @@ module.exports = class IncrementalTypescriptCompiler {
   outDir() {
     if (!this._outDir) {
       let outDir = path.join(tmpdir(), `e-c-ts-${process.pid}`);
+      fs.removeSync(outDir);
+
       this._outDir = outDir;
       fs.mkdirsSync(outDir);
     }