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/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');