build(bazel): Turning on strictPropertyInitialization for Angular. (#24572)

All errors for existing fields have been detected and suppressed with a
`!` assertion.

Issue/24571 is tracking proper clean up of those instances.

One-line change required in ivy/compilation.ts, because it appears that
the new syntax causes tsickle emitted node to no longer track their
original sourceFiles.

PR Close #24572

packages/core/src/application_ref.ts

@@ -383,7 +383,8 @@ export class ApplicationRef {
   /**
    * Returns an Observable that indicates when the application is stable or unstable.
    */
-  public readonly isStable: Observable<boolean>;
+  // TODO(issue/24571): remove '!'.
+  public readonly isStable !: Observable<boolean>;
 
   /** @internal */
   constructor(