Why is DUB not passing dll.def file to linker

[Igor via Digitalmars-d-learn]

I am using DUB 1.3.0. My folder structure is:
project
  dub.json
  source
    windll.d
    handmade.d
    dll.def
dub.json looks like this:

{
        "name": "handmade",
        "targetType": "dynamicLibrary",
        "targetPath": "build",
        "configurations": [
                {
                        "name": "internal",
                        "versions": ["HANDMADE_INTERNAL"]
                }
        ]
}

But when I run: dub -v build -ax86_64 --force
I get:

Generating using build
Generate target handmade (dynamicLibrary D:\git\temp\build 
handmade)
Performing "debug" build using dmd for x86_64.
handmade ~master: building configuration "internal"...
dmd -m64 -c 
-of.dub\build\internal-debug-windows-x86_64-dmd_2074-C56D6B49201C03F44B01E754688EACEE\handmade.obj -debug -g -w -version=HANDMADE_INTERNAL -version=Have_handmade -Isource source\handmade.d source\windll.d -vcolumns
Linking...
dmd 
-of.dub\build\internal-debug-windows-x86_64-dmd_2074-C56D6B49201C03F44B01E754688EACEE\handmade.dll .dub\build\internal-debug-windows-x86_64-dmd_2074-C56D6B49201C03F44B01E754688EACEE\handmade.obj -m64 -shared -g
Copying target from 
D:\git\temp\.dub\build\internal-debug-windows-x86_64-dmd_2074-C56D6B49201C03F44B01E754688EACEE\handmade.dll to D:\git\temp\build

There is no mention of dll.def file.