Currently when using a code editor that uses the TypeScript language service (Webstorm / VS Code / Atom / Sublime) there will be TypeScript errors in the `.spec` files.

This is because the editor finds the `tsconfig.json` file that doesn't match the spec files. Therefore it complains about the `experimentalDecorators`.

In `angular/angular` there is a global `tsconfig` file that matches everything and is therefore fixing the editor warnings. This file is also used to compile all `.spec` files for the whole project.

We can add such a `tsconfig.json` file as well, and in the future we can use it for compiling the specs as well.