How can I use Mono's xbuild to build a TypeScript project on Linux?

Question

I'm developing this project with Visual Studio on Windows, but I've had no problems compiling it with xbuild on Linux until I added some files that need to be built with TypeScript. Specifically, a set of .ts files are compiled to produce app.js, which is then included as an embedded resource. The build works in VS, but with xbuild I get this:

$ xbuild Guncho.sln
XBuild Engine Version 12.0
Mono, Version 3.10.0.0
Copyright (C) 2005-2013 Various Mono authors

Build started 12/23/2014 13:00:53.
__________________________________________________
Project "/home/guncho/test-guncho/Guncho.sln" (default target(s)):
        Target ValidateSolutionConfiguration:
                Building solution configuration "Debug|Any CPU".
        Target Build:
                Project "/home/guncho/test-guncho/Guncho.Site/Guncho.Site.csproj" (default target(s)):
                        Target PrepareForBuild:
                                Configuration: Debug Platform: AnyCPU
                        Target CopyNonResxEmbeddedResources:
/usr/lib/mono/xbuild/12.0/bin/Microsoft.Common.targets: error : Cannot copy /home/guncho/test-guncho/Guncho.Site/app/app.js to /home/guncho/test-guncho/Guncho.Site/obj/Debug/Guncho.Site.app.app.js, as the source file doesn't exist.
/usr/lib/mono/xbuild/12.0/bin/Microsoft.Common.targets: error : Cannot copy /home/guncho/test-guncho/Guncho.Site/app/app.js.map to /home/guncho/test-guncho/Guncho.Site/obj/Debug/Guncho.Site.app.app.js.map, as the source file doesn't exist.
                        Task "Copy" execution -- FAILED
                        Done building target "CopyNonResxEmbeddedResources" in project "/home/guncho/test-guncho/Guncho.Site/Guncho.Site.csproj".-- FAILED
                Done building project "/home/guncho/test-guncho/Guncho.Site/Guncho.Site.csproj".-- FAILED
        Task "MSBuild" execution -- FAILED
        Done building target "Build" in project "/home/guncho/test-guncho/Guncho.sln".-- FAILED
Done building project "/home/guncho/test-guncho/Guncho.sln".-- FAILED

Build FAILED.

It's not even attempting the TypescriptCompile step. The build works if I manually run tsc to generate app.js, but what do I need to do to get it working automatically?