build: turn on tsc's stripInternal when producint public d.ts file

I also made some changes to fix imports and remove some stuff that caused
breakage once stripInternals was turned on.

modules/@angular/common/src/directives/observable_list_diff.ts

@@ -1,11 +0,0 @@
-// TS does not have Observables
-
-// I need to be here to make TypeScript think this is a module.
-import {} from '../../src/facade/lang';
-
-/**
- * This module exists in Dart, but not in Typescript. This exported symbol
- * is only here to help Typescript think this is a module.
- * @internal
- */
-export var workaround_empty_observable_list_diff: any;