AngularJS compilation is a synchronous operation (unless having to fetch
a template, which is not supported for downgraded components).
Previously, ngUpgrade tried to retain the synchronous nature of the
compilation for downgraded components (when possible), by using a
synchronous thenable implementation (`ParentInjectorPromise`). This was
accidentally broken in #27217 by replacing a call to
`ParentInjectorPromise#then()` (which can be synchronous) with a call to
`Promise.all()` (which is asynchronous).
This commit fixes this by introducing a `SyncPromise.all()` static
method; similar to `Promise.all()` but retaining the synchronous
capabilities of `SyncPromise` (which `ParentInjectorPromise` inherits
from).
Fixes#30330
PR Close#31840
c1ae6124c8