source maps are surprisingly small

[devoncarew]

When used with the atom plugin, source maps for dart files are surprisingly small:

{"version":3,"sourceRoot":"","sources":[],"names":[],"mappings":"","file":"entry.js"}

It doesn't look like they have line number info.

[vsmenon]

CL out for this:

https://codereview.chromium.org/1425313003/

[devoncarew]

They're bigger now :)

{"version":3,"sourceRoot":"","sources":["../entry.dart"],"names":["print","registerPackage"],"mappings":";;;;;;;;;AAOA,kBAAO;AACL,IAAA,AAAAA,UAAK,CAAC,4BAA4B,CAAC;AAAC,AASpC,IAAA,AAAAC,oBAAe,CAAC,mCAAuB,CAAC;AAAC,GAE3C;;AAAA","file":"entry.js"}

Thanks!