require.resolve.paths(${built-in module id}) returns null

[shinnn]

• Version: v9.4.0
• Platform: macOS 10.13.2

When require.resolve.paths() receives a Node.js built-in module ID, for example fs and http2, it doesn't return Array<string> but null.

$ node -p "require.resolve.paths('.');"
[ '/Users/shinnn/test',
  '/Users/shinnn/test/node_modules',
  '/Users/shinnn/node_modules',
  '/Users/node_modules',
  '/node_modules',
  '/Users/shinnn/.node_modules',
  '/Users/shinnn/.node_libraries',
  '/usr/local/Cellar/node/9.4.0/lib/node' ]

$ node -p "require.resolve.paths('fs');"
null

$ node -p "require.resolve.paths('http2');"
null

Is this an expected behavior? I'm OK with the current null returning behavior but we should inform this special case in the API document. Otherwise, if the original intent of #16397 is always retrieving Array<string> as currently documented, we should fix the bug.

---

For reference, require.resolve returns the ID itself when it takes one of the built-in module IDs.

$  node -p "require.resolve('fs');"
fs

[cjihrig]

I think documenting null as a possible return value here is fine.

[shinnn]

@cjihrig @jasnell @nephross @vsemozhetbyt Thanks.