refactor(@angular-devkit/core): fix strict typing errors in node/

This fixes a few error surfaced by adding `"strict": true` to `tsconfig.json`.
* `_callFs(fs.readdir, /* ... */)` needs explicit types because `fs.readdir` has a few overloads, which was confusing type inference.
* `TempScopedNodeJsSyncHost._sync` is an uninitialized property, but it was already being checked for `undefined`, so I simply made its type optional.
* `TempScopedNodeJsSyncHost.files` had an incorrect type assertion, but was otherwise correct. I just removed the assertion and let type inference do the trick.

Author: dgp1130

etc/api/angular_devkit/core/node/testing/index.d.ts

@@ -1,6 +1,6 @@
 export declare class TempScopedNodeJsSyncHost extends virtualFs.ScopedHost<fs.Stats> {
     protected _root: Path;
-    protected _sync: virtualFs.SyncDelegateHost<fs.Stats>;
+    protected _sync?: virtualFs.SyncDelegateHost<fs.Stats>;
     get files(): Path[];
     get root(): Path;
     get sync(): virtualFs.SyncDelegateHost<fs.Stats>;

packages/angular_devkit/core/node/host.ts

@@ -160,8 +160,8 @@ export class NodeJsAsyncHost implements virtualFs.Host<fs.Stats> {
   }
 
   list(path: Path): Observable<PathFragment[]> {
-    return _callFs(fs.readdir, getSystemPath(path)).pipe(
-      map((names: string[]) => names.map(name => fragment(name))),
+    return _callFs<string[], string>(fs.readdir, getSystemPath(path)).pipe(
+      map((names) => names.map(name => fragment(name))),
     );
   }
 

packages/angular_devkit/core/node/testing/index.ts

@@ -15,7 +15,7 @@ import { NodeJsSyncHost } from '../host';
  * A Sync Scoped Host that creates a temporary directory and scope to it.
  */
 export class TempScopedNodeJsSyncHost extends virtualFs.ScopedHost<fs.Stats> {
-  protected _sync: virtualFs.SyncDelegateHost<fs.Stats>;
+  protected _sync?: virtualFs.SyncDelegateHost<fs.Stats>;
   protected _root: Path;
 
   constructor() {
@@ -30,8 +30,8 @@ export class TempScopedNodeJsSyncHost extends virtualFs.ScopedHost<fs.Stats> {
     const sync = this.sync;
     function _visit(p: Path): Path[] {
       return sync.list(p)
-        .map((fragment: PathFragment) => join(p, fragment))
-        .reduce((files: Path[], path: PathFragment) => {
+        .map((fragment) => join(p, fragment))
+        .reduce((files, path) => {
           if (sync.isDirectory(path)) {
             return files.concat(_visit(path));
           } else {