Assets not picked up with multi project roots

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

What is the current behavior?
If you have a project setup with multiple roots and there are overlapping relative directories, assets are only picked up from directory in the first root.

Here is a failing testcase:

      const server = new AssetServer({
        projectRoots: ['/root', '/root2'],
        assetExts: ['png'],
      });

      fs.__setMockFilesystem({
        'root': {
          imgs: {
            'a.png': 'a image',
          },
        },
        'root2': {
          imgs: {
            'b.png': 'b image',
          },
        },
      });

      return Promise.all([
        server.get('imgs/a.png').then(data =>
          expect(data).toBe('a image')
        ),
        server.get('imgs/b.png').then(data =>
          expect(data).toBe('b image')
        ),
      ]);

The test fails on not being able to find imgs/b.png, because internally _findRoot always returns the first matching path (imgs) and no b.png is present there.

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.

Checkout the fork at https://github.com/AlbertBrand/metro-bundler

I added the above testcase to AssetServer-test.js that shows the issue.

What is the expected behavior?
I would expect the asset server to look in each project root for the asset. Or, even more correct: the AssetServer get call should take in account the root of the asset. If you then would refer to an asset from a specific root, it can directly resolve the correct one.