Ambient types are not automatically discovered

[alexeagle]

By default, TypeScript discovers ambient types under node_modules/@types/* to be resolvable. For example, a .ts file can just contain import * as fs from 'fs' and it works if the node typings are installed.

Under bazel the types must be explicitly referenced, eg.
/// <reference types="node"/>

It doesn't help to list the types in the tsconfig compilerOptions.types[] which is not surprising since IIUC that only restricts which types the compiler includes in resolution.

We noticed this yesterday in the Angular team (cc @matsko and @IgorMinar), independently reported in #30.