Steps to reproduce:
1. Open attached project "HttpBug"
2. Make sure Scripting Runtime Version is set to .NET 4.x
3. Open "main" scene
4. Open "test" script with VS2017
5. Press Attach to Unity
-Notice "The type or namespace name 'HttpClient' could not be found" error appears

Note: using mcs.rsp file with contents "-r:System.Net.Http.dll" allows to run the script without problems in the editor but debugging is still not possible.

Actual result: VS can't find namespace "HttpClient"
Expected result: VS should be able to see "HttpClient" namespace without errors

Reproduced with: 2018.1.0a1, 2018.1.8f1, 2018.2.3f1, 2018.3.0a1, 2018.3.0a8
Didn't reproduce with: 2017.4.9f1
Regressed in: 2018.1.0a1

In 2018.3.0a8 msc.rsp workaround doesn't work (changing the file name to csc.rsp has no effect also)

Visual Studio version is 15.7.6
VSTU version is 3.7.0.1