* Revert "fix(d.ts): enable angular2 compilation with TS flag --strictNullChecks (#8902)"
This reverts commit 7e352a27f7172a80cfc5aefa6296ebbaca8df450.
* test: add typescript test for our typings
Eliminate the following compiler error when using TS 1.9+ with strict null checking enabled:
node_modules/@angular/core/src/util/decorators.d.ts(9,5): error TS2411: Property 'extends' of type 'Type | undefined' is not assignable to string index type 'Type | Function | any[]'.
https://github.com/angular/angular/issues/8720