Ads package is not picking up assembly reference for Advertisement namespace when "Ads Service" is enabled afterward

Steps to Repro:
1. Create a new or open attached project
2. Project window > right click
3. C# Script > write a code to use Ads or import the attached script

Actual Result:
1. Observe errors in the console
2. Advertisement namespace is not recognized when services are enabled afterward

Expected Result:
1. Once services are enabled " Advertisement" should be recognized
2. Compiler errors should disappear on enabling ads services

Workaround:
1. Downgrading and upgrading the ads package resolves the problem
2. Removing and installing the package back resolves the problem, however, if the script is re-imported the error appears again

Note:
1. Occurring with Ads 2.0.7 or 2.0.8
2. Ads Package is installed by default
3. Make sure ads are enabled in unity services
4. Integration of ads guide

https://unityads.unity3d.com/help/monetization/integration-guide-unity

Occurring with:
2018.3.0a4, 2018.2.0b10, 2018.1.0f2, 2018.1.0b13, 2018.1.0b5

Environment:
Windows & Mac

Stack Trace:
The name `Advertisement' does not exist in the current context