test(ivy): Add bazel flag to control building ViewEngine or Ivy (#23833)

PR Close #23833

packages/core/src/ivy_switch.ts

@@ -0,0 +1,34 @@
+/**
+ * @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
+ */
+
+/**
+ * This file is used to control if the default rendering pipeline should be `ViewEngine` or `Ivy`.
+ *
+ * Reexport from:
+ * - `./ivy_switch_false` => Use `ViewEngine`.
+ * - `./ivy_switch_true` => Use `Ivy`.
+ *
+ * This file is here for your IDE as well as for `google3`. The `bazel` build system
+ * specifically excludes this file and instead generates a new file which is controlled by
+ * command line:
+ *
+ * - `bazel build packages/core` => Use `ViewEngine`
+ * - `bazel build packages/core --define=ivy=true` => Use `Ivy`
+ *
+ * See: `bazel build packages/core:ivy_switch` for more details.
+ *
+ * ## How to use this
+ *
+ * Use this mechanism to have the same symbol be aliased to different implementation.
+ * 1) Create two implementations of a symbol (most likely a `function` or a `class`).
+ * 2) Export the two implementation under same name in `./ivy_switch_false` and `./ivy_switch_false`
+ *    respectively.
+ * 3) Import the symbol from `./ivy_switch`. The imported symbol will that point to either the
+ *    symbol in `./ivy_switch_false` and `./ivy_switch_false` depending on the compilation mode.
+ */
+export * from './ivy_switch_false';