fix: turn on nullability in the code base.

packages/core/public_api.ts

@@ -14,9 +14,3 @@
 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;
-}