"System.Net.Http.dll could not be found" error is thrown when msc.rsp/csc.rsp file with the content "-r:System.Net.Http.dll" is present in the Assets folder and Scripting Runtime Version is set to .NET 4.x
Steps to reproduce:
1. Open attached project
-error "CS0006: Metadata file 'System.Net.Http.dll' could not be found" is thrown in the console

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

Full error:
Microsoft (R) Visual C# Compiler version 2.7.0.65535 (ced8f62d)
Copyright (C) Microsoft Corporation. All rights reserved.
error CS0006: Metadata file 'System.Net.Http.dll' could not be found