Improve the readability of some human-consumed strings in source_maps.

R=jmesserly@google.com

Review URL: https://codereview.chromium.org//346983002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/source_maps@37530 260f80e4-7a28-3924-810f-c04153c831b5
3 files changed
tree: ddf0447c747f0d368d30c13eb0cf7e6d69dca27b
  1. pkgs/