fix(compiler-cli): expose the linker as a Babel plugin (#41918)

This allows the linker to be used as a true Babel plugin. In a Babel
configuration file, include the linker as follows:

```js
{
  plugins: [
    '@angular/compiler-cli/linker/babel',
  ]
}
```

or, if you need to specify configuration options:

```js
{
  plugins: [
    ['@angular/compiler-cli/linker/babel', {linkerJitMode: true}],
  ]
}
```

PR Close #41918

packages/compiler-cli/linker/babel/index.ts

@@ -5,4 +5,7 @@
  * 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
  */
-export {createEs2015LinkerPlugin} from './src/es2015_linker_plugin';
+import {defaultLinkerPlugin} from './src/babel_plugin';
+
+export {createEs2015LinkerPlugin} from './src/es2015_linker_plugin';
+export default defaultLinkerPlugin;

packages/compiler-cli/linker/babel/src/babel_plugin.ts

@@ -0,0 +1,39 @@
+/**
+ * @license
+ * Copyright Google LLC 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 {ConfigAPI, PluginObj} from '@babel/core';
+
+import {NodeJSFileSystem} from '../../../src/ngtsc/file_system';
+import {ConsoleLogger, LogLevel} from '../../../src/ngtsc/logging';
+import {LinkerOptions} from '../../src/file_linker/linker_options';
+
+import {createEs2015LinkerPlugin} from './es2015_linker_plugin';
+
+/**
+ * This is the Babel plugin definition that is provided as a default export from the package, such
+ * that the plugin can be used using the module specifier of the package. This is the recommended
+ * way of integrating the Angular Linker into a build pipeline other than the Angular CLI.
+ *
+ * When the module specifier `@angular/compiler-cli/linker/babel` is used as a plugin in a Babel
+ * configuration, Babel invokes this function (by means of the default export) to create the plugin
+ * instance according to the provided options.
+ *
+ * The linker plugin that is created uses the native NodeJS filesystem APIs to interact with the
+ * filesystem. Any logging output is printed to the console.
+ *
+ * @param api Provides access to the Babel environment that is configuring this plugin.
+ * @param options The plugin options that have been configured.
+ */
+export function defaultLinkerPlugin(api: ConfigAPI, options: Partial<LinkerOptions>): PluginObj {
+  api.assertVersion(7);
+
+  return createEs2015LinkerPlugin({
+    ...options,
+    fileSystem: new NodeJSFileSystem(),
+    logger: new ConsoleLogger(LogLevel.info),
+  });
+}

packages/compiler-cli/linker/babel/test/babel_plugin_spec.ts

@@ -0,0 +1,56 @@
+/**
+ * @license
+ * Copyright Google LLC 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 {transformSync} from '@babel/core';
+
+describe('default babel plugin entry-point', () => {
+  it('should work as a Babel plugin using the module specifier', () => {
+    const result = transformSync(
+        `
+        import * as i0 from "@angular/core";
+
+        export class MyMod {}
+        export class MyComponent {}
+
+        MyMod.ɵmod = i0.ɵɵngDeclareNgModule({ minVersion: "12.0.0", version: "0.0.0-PLACEHOLDER", ngImport: i0, type: MyMod, declarations: [MyComponent] });
+       `,
+        {
+          plugins: [
+            '@angular/compiler-cli/linker/babel',
+          ],
+          filename: 'test.js',
+        })!;
+
+    expect(result).not.toBeNull();
+    expect(result.code).not.toContain('ɵɵngDeclareNgModule');
+    expect(result.code).toContain('i0.ɵɵdefineNgModule');
+    expect(result.code).not.toMatch(/declarations:\s*\[MyComponent]/);
+  });
+
+  it('should be configurable', () => {
+    const result = transformSync(
+        `
+        import * as i0 from "@angular/core";
+
+        export class MyMod {}
+        export class MyComponent {}
+
+        MyMod.ɵmod = i0.ɵɵngDeclareNgModule({ minVersion: "12.0.0", version: "0.0.0-PLACEHOLDER", ngImport: i0, type: MyMod, declarations: [MyComponent] });
+       `,
+        {
+          plugins: [
+            ['@angular/compiler-cli/linker/babel', {linkerJitMode: true}],
+          ],
+          filename: 'test.js',
+        })!;
+
+    expect(result).not.toBeNull();
+    expect(result.code).not.toContain('ɵɵngDeclareNgModule');
+    expect(result.code).toContain('i0.ɵɵdefineNgModule');
+    expect(result.code).toMatch(/declarations:\s*\[MyComponent]/);
+  });
+});