Add columns to breakpoints

[annagrin]

This change allows setting breakpoints on dart lines that are compiled into the middle of longer JS lines. For example, in the following code Scaffold(...) is compiled into one JavaScript line, which did not let us set a breakpoint on // Breakpoint on this line line before:

Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar( // Breakpoint on this line
        title: Text(widget.title),
      ),
    ... 
   );
}

[nshahan]

Thanks Anna! LGTM with a few comments.