Generalize source entry

R=scheglov@google.com

Review URL: https://codereview.chromium.org//661603004

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@41151 260f80e4-7a28-3924-810f-c04153c831b5
It's not IE, but chrome that was slow.

Review URL: https://codereview.chromium.org//657393002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@41152 260f80e4-7a28-3924-810f-c04153c831b5