In Chrome, Tools -> Developer Tools -> Elements tab. There are no line numbers next to the source code. Is there a way to toggle them on?
If you go to View Page Source, this will show the current line numbers. Either right-click -> View Page Source, or hit Ctrl+U on Windows, or Cmd+Option+U on Mac.
For what it's worth, I needed this because I was getting a javascript error with a line number. The dev tools weren't showing this snippet in the regular source tab, presumably because it was embedded in HTML and I had a compilation error (stupid missing comma).