fix: prevent strictNullChecks support until #15432 is fixed (#15434)

packages/core/public_api.ts

@@ -14,3 +14,9 @@
 export * from './src/core';
 
 // This file only reexports content of the `src` folder. Keep it that way.
+
+// This is a hack to prevent people from turning on strictNullChecks. See #15432
+export declare interface ɵStrictNullChecksNotSupported {
+  dontUseStrictNullChecksWithAngularYetSeeIssue15432: string|null;
+  [key: string]: string;
+}