Change "FormatException.position" to be named "offset".

Address comments on SpanFormatException changes.

R=nweiz@google.com

Committed: https://code.google.com/p/dart/source/detail?r=38373

Committed: https://code.google.com/p/dart/source/detail?r=38378

Review URL: https://codereview.chromium.org//396603003

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/source_maps@38379 260f80e4-7a28-3924-810f-c04153c831b5