Compile error with --strict-null-checks enabled.

[mhverbakel]

Strict null checking was added by microsoft/TypeScript#7140. This allows developers to make their code more type-safe by explicitly defining null or undefined as a type option.

This library gives me an compile error if strict null checking is enabled:

node_modules/await-semaphore/index.ts(12,13): error TS2532: Object is possibly 'undefined'.

Steps to reproduce

1. Create a typescript 2.0 project with strict null checking enabled
2. Add await-semaphore as a dependency
3. Run tsc.