refactor(ivy): ngcc - implement abstract FileSystem (#29643)

This commit introduces a new interface, which abstracts access
to the underlying `FileSystem`. There is initially one concrete
implementation, `NodeJsFileSystem`, which is simply wrapping the
`fs` library of NodeJs.

Going forward, we can provide a `MockFileSystem` for test, which
should allow us to stop using `mock-fs` for most of the unit tests.
We could also implement a `CachedFileSystem` that may improve the
performance of ngcc.

PR Close #29643

packages/compiler-cli/ngcc/src/file_system/file_system.ts

@@ -0,0 +1,38 @@
+/**
+ * @license
+ * Copyright Google Inc. All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+
+import {AbsoluteFsPath, PathSegment} from '../../../src/ngtsc/path';
+
+/**
+ * A basic interface to abstract the underlying file-system.
+ *
+ * This makes it easier to provide mock file-systems in unit tests,
+ * but also to create clever file-systems that have features such as caching.
+ */
+export interface FileSystem {
+  exists(path: AbsoluteFsPath): boolean;
+  readFile(path: AbsoluteFsPath): string;
+  writeFile(path: AbsoluteFsPath, data: string): void;
+  readdir(path: AbsoluteFsPath): PathSegment[];
+  lstat(path: AbsoluteFsPath): FileStats;
+  stat(path: AbsoluteFsPath): FileStats;
+  pwd(): AbsoluteFsPath;
+  copyFile(from: AbsoluteFsPath, to: AbsoluteFsPath): void;
+  moveFile(from: AbsoluteFsPath, to: AbsoluteFsPath): void;
+  ensureDir(path: AbsoluteFsPath): void;
+}
+
+/**
+ * Information about an object in the FileSystem.
+ * This is analogous to the `fs.Stats` class in Node.js.
+ */
+export interface FileStats {
+  isFile(): boolean;
+  isDirectory(): boolean;
+  isSymbolicLink(): boolean;
+}

packages/compiler-cli/ngcc/src/file_system/node_js_file_system.ts

@@ -0,0 +1,30 @@
+/**
+ * @license
+ * Copyright Google Inc. All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+
+import * as fs from 'fs';
+import {cp, mkdir, mv} from 'shelljs';
+import {AbsoluteFsPath, PathSegment} from '../../../src/ngtsc/path';
+import {FileSystem} from './file_system';
+
+/**
+ * A wrapper around the Node.js file-system (i.e the `fs` package).
+ */
+export class NodeJSFileSystem implements FileSystem {
+  exists(path: AbsoluteFsPath): boolean { return fs.existsSync(path); }
+  readFile(path: AbsoluteFsPath): string { return fs.readFileSync(path, 'utf8'); }
+  writeFile(path: AbsoluteFsPath, data: string): void {
+    return fs.writeFileSync(path, data, 'utf8');
+  }
+  readdir(path: AbsoluteFsPath): PathSegment[] { return fs.readdirSync(path) as PathSegment[]; }
+  lstat(path: AbsoluteFsPath): fs.Stats { return fs.lstatSync(path); }
+  stat(path: AbsoluteFsPath): fs.Stats { return fs.statSync(path); }
+  pwd() { return AbsoluteFsPath.fromUnchecked(process.cwd()); }
+  copyFile(from: AbsoluteFsPath, to: AbsoluteFsPath): void { cp(from, to); }
+  moveFile(from: AbsoluteFsPath, to: AbsoluteFsPath): void { mv(from, to); }
+  ensureDir(path: AbsoluteFsPath): void { mkdir('-p', path); }
+}