This checks for a Bazel flag in `ng_module()` in the `_renderer` attribute
which specifies the renderer to use for the build.
The main advantage of this flag is that it can be overridden with [Bazel
transitions](https://docs.bazel.build/versions/master/skylark/config.html),
giving much more flexibility for migrating individual applications in a
Bazel workspace to Ivy.
This flag is not intended to replace `--config ivy` or
`--define angular_ivy_enabled=True` (although it technically could). As a
result, this flag is not and will not actually be used anywhere in the
`angular/angular` repo. Instead, a `string_flag()` is provided internally
which sets the renderer via a transition. See http://cl/315749946.
Note that this does **not** introduce a dependency on Skylib for
`angular/angular`. The dependency isn't actually necessary because
`BuildSettingInfo` is not used externally anyways. By doing this, it is not
necessary for downstream, external workspaces to depend on Skylib.
PR Close#37529
1091ddbb8e