-e: see title, worked in beta18 now fails on OSX and Win

-repro:
--create new empty project (or download the attached one)
--start a WebGL build
--NOTICE it fails with build errors:
---

Failed running D:\Unity Installs\5.4.0b19\Editor\Data\il2cpp/build/il2cpp.exe --convert-to-cpp --compile-cpp --libil2cpp-static --platform="WebGL" --architecture="EmscriptenJavaScript" --configuration="Release" --outputpath="D:/Work/Bugs/New Unity Project 1/Assets /../Temp/StagingArea/Data\Native\UserAssembly.bc" --cachedirectory="D:/Work/Bugs/New Unity Project 1/Assets/../Library/il2cpp_cache" --compiler-flags="-Oz -DIL2CPP_EXCEPTION_DISABLED=1 " --linker-flags="-Oz -s NO_EXIT_RUNTIME=1" --additional-include-directories="D:/Work/Bugs/New Unity Project 1/Assets/../Temp/StagingArea/Data\Native\UserAssembly.bc" --extra-types.file="D:\Unity Installs\5.4.0b19\Editor\Data\il2cpp\il2cpp_default_extra_types.txt" --assembly="D:\Work\Bugs\New Unity Project 1\Temp\StagingArea\Data\Managed\UnityEngine.dll" --generatedcppdir="D:\Work\Bugs\New Unity Project 1\Temp\StagingArea\Data\il2cppOutput"

stdout:
Building UserAssembly.bc with EmscriptenToolChain.
Output directory: D:\Work\Bugs\New Unity Project 1\Temp\StagingArea\Data\Native
Cache directory: D:\Work\Bugs\New Unity Project 1\Library\il2cpp_cache
il2cpp.exe didn't catch exception: System.InvalidOperationException: Builder is unable to build using selected toolchain (EmscriptenToolChain) or architecture (Unity.IL2CPP.Building.EmscriptenJavaScriptArchitecture)!
at Unity.IL2CPP.Building.CppProgramBuilder.Build()
at il2cpp.Program.DoRun(String[] args)
at il2cpp.Program.Run(String[] args)
at il2cpp.Program.Main(String[] args)
stderr:

Unhandled Exception: System.InvalidOperationException: Builder is unable to build using selected toolchain (EmscriptenToolChain) or architecture (Unity.IL2CPP.Building.EmscriptenJavaScriptArchitecture)!
at Unity.IL2CPP.Building.CppProgramBuilder.Build()
at il2cpp.Program.DoRun(String[] args)
at il2cpp.Program.Run(String[] args)
at il2cpp.Program.Main(String[] args)

UnityEngine.Debug:LogError(Object)
UnityEditorInternal.Runner:RunManagedProgram(String, String, String, CompilerOutputParserBase, Action`1) (at C:/buildslave/unity/build/Editor/Mono/BuildPipeline/Il2Cpp/IL2CPPUtils.cs:98)
UnityEditorInternal.IL2CPPBuilder:ConvertPlayerDlltoCpp(ICollection`1, String, String) (at C:/buildslave/unity/build/Editor/Mono/BuildPipeline/Il2Cpp/IL2CPPUtils.cs:350)
UnityEditorInternal.IL2CPPBuilder:Run() (at C:/buildslave/unity/build/Editor/Mono/BuildPipeline/Il2Cpp/IL2CPPUtils.cs:213)
UnityEditorInternal.IL2CPPUtils:RunIl2Cpp(String, IIl2CppPlatformProvider, Action`1, RuntimeClassRegistry, Boolean) (at C:/buildslave/unity/build/Editor/Mono/BuildPipeline/Il2Cpp/IL2CPPUtils.cs:139)
UnityEditor.HostView:OnGUI()