Asset paths (httpServerLocation) invalid when asset is in watchFolders

Do you want to request a feature or report a bug?
Bug

What is the current behavior?
When I specify a directory in watchFolders from outside of my project root, the path reported via httpServerLocation is broken. For example, /symlinked-watch-folder/subdir/image@3x.png.

If the current behavior is a bug, please provide the steps to reproduce and a minimal repository on GitHub that we can yarn install and yarn test.

1. Create a React Native project in one directory.
2. Create a sibling directory called symlinked-watch-folder with a Node module in it named symlinked-module.
3. Add assets to this module, specifically one in subdir/image@3x.png.
4. In this module, reference the image like so: <Image source={require('../subdir/image.png')} />
5. Now from the RN project directory, use npm to install the module: npm install ../symlinked-watch-folder.
6. Make a component in your project root that displays the module's component with the image in it.
7. Run the React Native project. The image will not appear and a request for the image will 404 because the path is invalid.

What is the expected behavior?
It looks like there are different valid locations, including:
/assets/symlinked-module/subdir/image@3x.png
/assets/subdir/image@3x.png
/subdir/image@3x.png

More info
Because this repo isn't very publicly documented, I can't tell exactly what's going on. But it looks like getAssetData is trying to path.join these two paths:
/assets
../symlinked-watch-folder/subdir/image@3x.png
https://github.com/facebook/metro/blob/bba48f068c6e709187d36170d3e5d49b7e9f6169/packages/metro/src/Assets.js#L181
The ../ wipes out the /assets and we end up with a result that isn't valid:
/symlinked-watch-folder/subdir/image@3x.png

I believe the relative path is coming from here:
https://github.com/facebook/metro/blob/bba48f068c6e709187d36170d3e5d49b7e9f6169/packages/metro/src/DeltaBundler/Serializers/getAssets.js#L44

The asset is being served correctly, it's just that the httpServerLocation that's being generated is not correct.
I can access the asset just fine if I correct the path to one of these:
/assets/symlinked-module/subdir/image@3x.png
/assets/subdir/image@3x.png
/subdir/image@3x.png

If there is some way to handle this using the documented options, please let me know. (cc: @rafeca )

Please provide your exact Metro configuration and mention your Metro, node, yarn/npm version and operating system.
I've tried both Metro 0.47.1 and Metro 0.48.0
Node: 10.11.0
npm: 6.4.1
OS: macOS High Sierra

Start with packages/metro/src/Assets.js, especially getAssetData and the httpServerLocation generation, then compare it with packages/metro/src/DeltaBundler/Serializers/getAssets.js. Reproduce the watchFolders case using the listed sibling-directory setup and verify the generated asset URL serves the image instead of returning 404.