OK, let's port following libraries from Guava:
1. Precondition#check*() methods.
2. VisibleForTesting annotation.

What do you think about Nullable annotation? We can find it from following pom:

```
      <dependency>
        <groupId>com.google.code.findbugs</groupId>
        <artifactId>jsr305</artifactId>
        <version>2.0.3</version>
      </dependency>
```