most source map paths are off by one

[devoncarew]

Here's my setup:

• most dart code is in the lib/ folder
• a single entry-point file is in web/ (web/entry.dart)
• all DDC output written to web/ddc/
• a DDC entry-point file is generated in web/entry_all.js which basically requires all the generated DDC sources in web/ddc/

Most of the generated source maps have incorrect paths.

web/ddc/entry.js.map has ../entry.dart for sources: (which is correct)

For package refs, web/ddc/logging/logging.js.map has ../../../logging/logging.dart. This is off-by-one; it should point to ../../../packages/logging/logging.dart.

For self-refs, web/ddc/atom_flutter/state.js.map points to ../../../atom_flutter/state.dart. It should probably point to ../../../lib/state.dart, or perhaps to ../../../packages/atom_flutter/state.dart.

I'm going to take a quick look through the source to see if the problem jumps out at me.