To reproduce:
1. Open QA attached project TestsetLightDirty
2. Build and run
3. See console for errors

Actual result: the error appears
Expected result: there should be no error and the build should be completed

Reproduced in: 2017.4.29f1; 2018.4.3f1; 2019.1.7f1; 2019.2.0b6; 2019.3.0a6

Error: Assets\TestSetLightDirty.cs(11,9): error CS1061: 'Light' does not contain a definition for 'SetLightDirty' and no accessible extension method 'SetLightDirty' accepting a first argument of type 'Light' could be found (are you missing a using directive or an assembly reference?)

The code:
void Start()
{
Light light = gameObject.GetComponent<Light>();
light.SetLightDirty();
}