fix(bazel): disable treeshaking when generating FESM and UMD bundles (#…

…32069)

There has been a regression where enabling rollup treeshaking causes errors during runtime because it will drop const access which will always evaluate to true or false. However, such `const` in `@angular/core` cannot be dropped because their value is changed when NGCC is run on `@angular/core`

VE
```
const SWITCH_IVY_ENABLED__POST_R3__ = true;
const SWITCH_IVY_ENABLED__PRE_R3__ = false;
const ivyEnabled = SWITCH_IVY_ENABLED__PRE_R3__;
```

Ivy (After NGCC)
```
const SWITCH_IVY_ENABLED__POST_R3__ = true;
const SWITCH_IVY_ENABLED__PRE_R3__ = false;
const ivyEnabled = SWITCH_IVY_ENABLED__POST_R3__;
```

FESM2015
```
load(path) {
	/** @type {?} */
	const legacyOfflineMode = this._compiler instanceof Compiler;
	return legacyOfflineMode ? this.loadFactory(path) : this.loadAndCompile(path);
}
```

ESM2015
```
 load(path) {
	/** @type {?} */
	const legacyOfflineMode = !ivyEnabled && this._compiler instanceof Compiler;
	return legacyOfflineMode ? this.loadFactory(path) : this.loadAndCompile(path);
}
```

From the above we can see that `ivyEnabled ` is being treeshaken away when generating the FESM bundle which is causing runtime errors such as `Cannot find module './lazy/lazy.module.ngfactory'` since in Ivy we will always load the factories.

PR Close #32069

packages/bazel/src/ng_package/ng_package.bzl

@@ -112,11 +112,13 @@ def _rollup(ctx, bundle_name, rollup_config, entry_point, inputs, js_output, for
         args.add("--amd.id", package_name)
 
     # After updating to build_bazel_rules_nodejs 0.27.0+, rollup has been updated to v1.3.1
-    # which tree shakes @__PURE__ annotations by default. We turn this feature off
-    # for ng_package as Angular bundles contain these annotations and there are
-    # test failures if they are removed. See comments in
-    # https://github.com/angular/angular/pull/29210 for more information.
-    args.add("--no-treeshake.annotations")
+    # which tree shakes @__PURE__ annotations and const variables which are later ammended by NGCC.
+    # We turn this feature off for ng_package as Angular bundles contain these and there are
+    # test failures if they are removed.
+    # See comments in:
+    # https://github.com/angular/angular/pull/29210
+    # https://github.com/angular/angular/pull/32069
+    args.add("--no-treeshake")
 
     # Note: if the input has external source maps then we need to also install and use
     #   `rollup-plugin-sourcemaps`, which will require us to use rollup.config.js file instead

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

@@ -133,6 +133,10 @@ describe('@angular/core ng_package', () => {
       } else {
         it('should have decorators',
            () => { expect(shx.cat('fesm5/core.js')).toContain('__decorate'); });
+
+        // See: https://github.com/angular/angular/pull/32069
+        it('should retain access to const',
+           () => { expect(shx.cat('fesm5/core.js')).toContain('!ivyEnabled'); });
       }
 
       it('should load tslib from external bundle', () => {