refactor(compiler-cli): linker - add Babel plugin, FileLinker and initial PartialLinkers (#39116)

This commit adds the basic building blocks for linking partial declarations.
In particular it provides a generic `FileLinker` class that delegates to
a set of (not yet implemented) `PartialLinker` classes.

The Babel plugin makes use of this `FileLinker` providing concrete classes
for `AstHost` and `AstFactory` that work with Babel AST. It can be created
with the following code:

```ts
const plugin = createEs2015LinkerPlugin({ /* options */ });
```

PR Close #39116

packages/compiler-cli/linker/test/file_linker/translator_spec.ts

@@ -0,0 +1,47 @@
+/**
+ * @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 * as o from '@angular/compiler';
+import {ImportGenerator, NamedImport, TypeScriptAstFactory} from '@angular/compiler-cli/src/ngtsc/translator';
+import * as ts from 'typescript';
+
+import {Translator} from '../../src/file_linker/translator';
+import {generate} from './helpers';
+
+describe('Translator', () => {
+  let factory: TypeScriptAstFactory;
+  beforeEach(() => factory = new TypeScriptAstFactory());
+
+  describe('translateExpression()', () => {
+    it('should generate expression specific output', () => {
+      const translator = new Translator<ts.Statement, ts.Expression>(factory);
+      const outputAst = new o.WriteVarExpr('foo', new o.LiteralExpr(42));
+      const translated = translator.translateExpression(outputAst, new MockImportGenerator());
+      expect(generate(translated)).toEqual('(foo = 42)');
+    });
+  });
+
+  describe('translateStatement()', () => {
+    it('should generate statement specific output', () => {
+      const translator = new Translator<ts.Statement, ts.Expression>(factory);
+      const outputAst = new o.ExpressionStatement(new o.WriteVarExpr('foo', new o.LiteralExpr(42)));
+      const translated = translator.translateStatement(outputAst, new MockImportGenerator());
+      expect(generate(translated)).toEqual('foo = 42;');
+    });
+  });
+  class MockImportGenerator implements ImportGenerator<ts.Expression> {
+    generateNamespaceImport(moduleName: string): ts.Expression {
+      return factory.createLiteral(moduleName);
+    }
+    generateNamedImport(moduleName: string, originalSymbol: string): NamedImport<ts.Expression> {
+      return {
+        moduleImport: factory.createLiteral(moduleName),
+        symbol: originalSymbol,
+      };
+    }
+  }
+});