100% code coverage for the preconditions module.
Usage:
```
$.checkState(something === anotherthing, 'Expected something to be
anotherthing');
$.checkArgument(something < 100, 'something', 'must be less than 100');
$.checkArgumentType(something, PrivateKey, 'something'); // The third
argument is a helper to mention the name of the argument
$.checkArgumentType(something, PrivateKey); // but it's optional (will
show up as "(unknown argument)")
```
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