Cross-targeted incremental unchanged builds are slow

[jeromelaban]

When having a simple cross-targeted project, such as this one:

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <TargetFrameworks>netcoreapp1.1;net46</TargetFrameworks>
  </PropertyGroup>
</Project>

Hitting F5 always re-executes a lot of msbuild targets even if nothing has changed. For this simple project, the top-level targets are :

1>      196 ms  ResolveAssemblyReference                   2 calls
1>      365 ms  ResolvePackageDependencies                 2 calls

This makes building large solutions that use cross-targeting very slow, as all dependent assemblies are partially re-evaluated. This gets worse if the number of packages installed is large.

[jeromelaban]

Tracked here dotnet/sdk#1116