feat(ivy): generate .ngsummary.js shims (#26495)

This commit adds generation of .ngsummary.js shims alongside .ngfactory.js
shims when generated files are enabled.

Generated .ngsummary shims contain a single, null export for every exported
class with decorators that exists in the original source files. Ivy code
does not depend on summaries, so these exist only as a placeholder to allow
them to be imported and their values passed to old APIs. This preserves
backwards compatibility.

Testing strategy: this commit adds a compiler test to verify the correct
shape and contents of the generated .ngsummary.js files.

PR Close #26495

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

@@ -10,3 +10,4 @@
 
 export {FactoryGenerator, FactoryInfo, generatedFactoryTransform} from './src/factory_generator';
 export {GeneratedShimsHostWrapper} from './src/host';
+export {SummaryGenerator} from './src/summary_generator';

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

@@ -0,0 +1,61 @@
+/**
+ * @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 ts from 'typescript';
+
+import {ShimGenerator} from './host';
+import {isNonDeclarationTsFile} from './util';
+
+export class SummaryGenerator implements ShimGenerator {
+  private constructor(private map: Map<string, string>) {}
+
+  getSummaryFileNames(): string[] { return Array.from(this.map.keys()); }
+
+  getOriginalSourceOfShim(fileName: string): string|null { return this.map.get(fileName) || null; }
+
+  generate(original: ts.SourceFile, genFilePath: string): ts.SourceFile {
+    // Collect a list of classes that need to have factory types emitted for them. This list is
+    // overly broad as at this point the ts.TypeChecker has not been created and so it can't be used
+    // to semantically understand which decorators are Angular decorators. It's okay to output an
+    // overly broad set of summary exports as the exports are no-ops anyway, and summaries are a
+    // compatibility layer which will be removed after Ivy is enabled.
+    const symbolNames = original
+                            .statements
+                            // Pick out top level class declarations...
+                            .filter(ts.isClassDeclaration)
+                            // which are named, exported, and have decorators.
+                            .filter(
+                                decl => isExported(decl) && decl.decorators !== undefined &&
+                                    decl.name !== undefined)
+                            // Grab the symbol name.
+                            .map(decl => decl.name !.text);
+
+    const varLines = symbolNames.map(name => `export const ${name}NgSummary: any = null;`);
+
+    if (varLines.length === 0) {
+      // In the event there are no other exports, add an empty export to ensure the generated
+      // summary file is still an ES module.
+      varLines.push(`export const ɵempty = null;`);
+    }
+    const sourceText = varLines.join('\n');
+    return ts.createSourceFile(
+        genFilePath, sourceText, original.languageVersion, true, ts.ScriptKind.TS);
+  }
+
+  static forRootFiles(files: ReadonlyArray<string>): SummaryGenerator {
+    const map = new Map<string, string>();
+    files.filter(sourceFile => isNonDeclarationTsFile(sourceFile))
+        .forEach(sourceFile => map.set(sourceFile.replace(/\.ts$/, '.ngsummary.ts'), sourceFile));
+    return new SummaryGenerator(map);
+  }
+}
+
+function isExported(decl: ts.Declaration): boolean {
+  return decl.modifiers !== undefined &&
+      decl.modifiers.some(mod => mod.kind == ts.SyntaxKind.ExportKeyword);
+}