message: refactor(compiler-cli): return the `FileSystem` from `initMockFileSystem()`
  It is common to want to use the file system once it is initialized,
  so it makes sense for this function to return it.
  PR Close
diff: @@ -66,10 +66,11 @@ runInEachFileSystem.unix = (callback: (os: string) => void) =>
  runInEachFileSystem.windows = (callback: (os: string) => void) =>
  runInFileSystem(FS_WINDOWS, callback, true);
  -export function initMockFileSystem(os: string, cwd?: AbsoluteFsPath): void {
  +export function initMockFileSystem(os: string, cwd?: AbsoluteFsPath): MockFileSystem {
  const fs = createMockFileSystem(os, cwd);
  setFileSystem(fs);
  monkeyPatchTypeScript(os, fs);
  + return fs;
  }
  function createMockFileSystem(os: string, cwd?: AbsoluteFsPath): MockFileSystem {
  