Azure Function Projects Do Not Support Incremental Builds

When I do a CTRL-SHIFT-B to build my solution, all projects that reference `Microsoft.NET.Sdk.Functions` get included in the build output every time. Is this a known issue? I did look around and nothing obvious turned up, so apologies if this is already captured somewhere.

This is my generated output in a solution with 20 projects. Do note that I am on .NET Core Preview 2:

```
1>------ Build started: Project: Project.Services.Application, Configuration: Release Any CPU ------
1>You are working with a preview version of the .NET Core SDK. You can define the SDK version via a global.json file in the current project. More at https://go.microsoft.com/fwlink/?linkid=869452
1>Project.Services.Application -> D:\Project\Project.Services.Application\bin\Release\netstandard2.0\bin\Project.Services.Application.dll
========== Build: 1 succeeded, 0 failed, 20 up-to-date, 0 skipped ==========
```