feat(ivy): generate flat module index files (#27497)

Previously, ngtsc did not respect the angularCompilerOptions settings
for generating flat module indices. This commit adds a
FlatIndexGenerator which is used to implement those options.

FW-738 #resolve

PR Close #27497

packages/compiler-cli/src/ngtsc/shims/index.ts

@@ -9,5 +9,6 @@
 /// <reference types="node" />
 
 export {FactoryGenerator, FactoryInfo, generatedFactoryTransform} from './src/factory_generator';
-export {GeneratedShimsHostWrapper} from './src/host';
+export {FlatIndexGenerator} from './src/flat_index_generator';
+export {GeneratedShimsHostWrapper, ShimGenerator} from './src/host';
 export {SummaryGenerator} from './src/summary_generator';

packages/compiler-cli/src/ngtsc/shims/src/flat_index_generator.ts

@@ -0,0 +1,70 @@
+/**
+ * @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 path from 'path';
+import * as ts from 'typescript';
+
+import {ShimGenerator} from './host';
+import {isNonDeclarationTsFile} from './util';
+
+export class FlatIndexGenerator implements ShimGenerator {
+  readonly flatIndexPath: string;
+
+  private constructor(
+      relativeFlatIndexPath: string, readonly entryPoint: string,
+      readonly moduleName: string|null) {
+    this.flatIndexPath = path.posix.join(path.posix.dirname(entryPoint), relativeFlatIndexPath)
+                             .replace(/\.js$/, '') +
+        '.ts';
+  }
+
+  static forRootFiles(flatIndexPath: string, files: ReadonlyArray<string>, moduleName: string|null):
+      FlatIndexGenerator|null {
+    // If there's only one .ts file in the program, it's the entry. Otherwise, look for the shortest
+    // (in terms of characters in the filename) file that ends in /index.ts. The second behavior is
+    // deprecated; users should always explicitly specify a single .ts entrypoint.
+    const tsFiles = files.filter(isNonDeclarationTsFile);
+    if (tsFiles.length === 1) {
+      return new FlatIndexGenerator(flatIndexPath, tsFiles[0], moduleName);
+    } else {
+      let indexFile: string|null = null;
+      for (const tsFile of tsFiles) {
+        if (tsFile.endsWith('/index.ts') &&
+            (indexFile === null || tsFile.length <= indexFile.length)) {
+          indexFile = tsFile;
+        }
+      }
+      if (indexFile !== null) {
+        return new FlatIndexGenerator(flatIndexPath, indexFile, moduleName);
+      } else {
+        return null;
+      }
+    }
+  }
+
+  recognize(fileName: string): boolean { return fileName === this.flatIndexPath; }
+
+  generate(): ts.SourceFile {
+    const relativeEntryPoint = './' +
+        path.posix.relative(path.posix.dirname(this.flatIndexPath), this.entryPoint)
+            .replace(/\.tsx?$/, '');
+
+    const contents = `/**
+ * Generated bundle index. Do not edit.
+ */
+
+export * from '${relativeEntryPoint}';
+`;
+    const genFile = ts.createSourceFile(
+        this.flatIndexPath, contents, ts.ScriptTarget.ES2015, true, ts.ScriptKind.TS);
+    if (this.moduleName !== null) {
+      genFile.moduleName = this.moduleName;
+    }
+    return genFile;
+  }
+}