build: enable importHelpers in tsconfig (#22812)

This is the primary tsconfig file used for Bazel builds.
Previously, we enabled this option only for releases.

PR Close #22812

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

@@ -143,6 +143,9 @@ describe('@angular/core ng_package', () => {
         expect(shx.cat('bundles/core.umd.js'))
             .toMatch(/@license Angular v\d+\.\d+\.\d+(?!-PLACEHOLDER)/);
       });
+
+      it('should have tslib helpers',
+         () => { expect(shx.cat('bundles/core.umd.js')).not.toContain('undefined.__extends'); });
     });
   });