Visual Studio: How to change target framework used by editor?

I have a multi-platform solution with many csproj files configured this way:

<PropertyGroup>
  <TargetFrameworks>net452;netstandard1.4</TargetFrameworks>    
</PropertyGroup>

<ItemGroup Condition=" '$(TargetFramework)' == 'netstandard1.4' ">    
  <PackageReference Include="System.Net.Http" Version="4.3.0"/>
  <PackageReference Include="Newtonsoft.Json" Version="9.0.1" />
</ItemGroup>

<ItemGroup Condition=" '$(TargetFramework)' == 'net452' ">
  <Reference Include="System.Net" />
  <Reference Include="System.Net.Http" />
  <PackageReference Include="Newtonsoft.Json" Version="6.0.8" />
</ItemGroup>

Note that I don't have a TargetFramework element (the singular form). I only have the TargetFrameworks element (the plural form).

The editor assumes netstandard1.4 and all #if NET452 blocks get grayed out and without IntelliSense.

How can I tell Visual Studio editor to recognize a specific target framework (net452, netstandard1.4), and get IntelliSense, at least temporarily?

For reference, I'm using VS2017 Professional version 15.2 (26430.16).