If any's interested, and assuming someone can upload a reasonably large javascript file as a module, it turns out this is very easy to accomplish with Bridge.net.

Step 1: Get Bridge.net for Visual Studio

Step 2: Create a Bridge.net project in your solution

Step 3: Edit the project file

Step 4: Add links to the source files from your other modules.  This can be accomplished en masse by using items of the following shape:

    <Compile Include="relative_project_root\**\*.cs" Exclude="relative_solution_root\**\AssemblyInfo.cs">
      <Link>folder_in_this_project\%(RecursiveDir)%(FileName)</Link>
    </Compile>

 Step 5: Iteratively compile and make sure your code cross compiles correctly.

I haven't really thought about cross-compiling my tests and seeing if they pass.  I probably should but I'm going to go out on a limb and guess that Bridge.net isn't going to handle the MsTest --> JsUnit (or whatever) conversion very well.

They don't support C# 7.0 but you still get approximately 6.0 more C#s than nothing.