Resolved in PR #1050

Description

ActionAssertions.NotThrowAfter and FunctionAssertions.NotThrowAfter are not available for .NET Standard 1.3 and 1.6.

This appears to be by design, due to conditional compilation logic which checks the target version and disallows compilation of these methods for those versions. It seems to be due to Thread.Sleep not being available, however this can be easily replaced with Task.Delay.

I also noticed that unit tests for NotThrowAfter were not being run for .NET Core 1.1. This should also be resolved.