fix(bazel): downlevel decorators in fesm5 files (#23078)

Needed so that our bundles appear side-effect-free to tools like webpack

PR Close #23078

packages/bazel/src/modify_tsconfig.js

@@ -23,10 +23,13 @@ function main(args) {
   const data = JSON.parse(fs.readFileSync(input, {encoding: 'utf-8'}));
   data['compilerOptions']['target'] = 'es5';
   data['bazelOptions']['es5Mode'] = true;
-  data['bazelOptions']['tsickle'] = false;
+  // Enable tsickle for decorator downleveling only
+  data['bazelOptions']['tsickle'] = true;
   data['bazelOptions']['tsickleExternsPath'] = '';
   data['compilerOptions']['outDir'] = path.join(data['compilerOptions']['outDir'], newRoot);
   if (data['angularCompilerOptions']) {
+    // Don't enable tsickle's closure conversions
+    data['angularCompilerOptions']['annotateForClosureCompiler'] = false;
     data['angularCompilerOptions']['expectedOut'] =
         data['angularCompilerOptions']['expectedOut'].map(
             f => f.replace(/\.closure\.js$/, '.js').replace(binDir, path.join(binDir, newRoot)));

packages/bazel/src/ngc-wrapped/index.ts

@@ -172,6 +172,8 @@ export function compile({allowNonHermeticReads, allDepsCompiledWithBazel = true,
   const bazelHost = new CompilerHost(
       files, compilerOpts, bazelOpts, tsHost, fileLoader, allowNonHermeticReads,
       generatedFileModuleResolver);
+  // Prevent tsickle adding any types at all if we don't want closure compiler annotations.
+  bazelHost.transformTypesToClosure = compilerOpts.annotateForClosureCompiler;
   const origBazelHostFileExist = bazelHost.fileExists;
   bazelHost.fileExists = (fileName: string) => {
     if (NGC_ASSETS.test(fileName)) {

packages/bazel/test/ng_package/core_package.spec.ts

@@ -126,6 +126,9 @@ describe('@angular/core ng_package', () => {
         expect(shx.cat('fesm5/core.js')).not.toContain('@fileoverview added by tsickle');
       });
 
+      it('should have annotations rather than decorators',
+         () => { expect(shx.cat('fesm5/core.js')).not.toContain('__decorate'); });
+
       it('should load tslib from external bundle', () => {
         expect(shx.cat('fesm5/core.js')).not.toContain('function __extends');
         expect(shx.cat('fesm5/core.js')).toMatch('import {.*__extends');