Extract out a source_span package from source_maps.

This is just the first step; future CLs will add support for the new API to
various packages currently using the old one.

BUG=19930
R=sigmund@google.com

Review URL: https://codereview.chromium.org//381363002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/source_span@38360 260f80e4-7a28-3924-810f-c04153c831b5
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_span@38379 260f80e4-7a28-3924-810f-c04153c831b5
Use a more compact internal representation for FileSpan.

R=rnystrom@google.com

Review URL: https://codereview.chromium.org//706133002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/source_span@41591 260f80e4-7a28-3924-810f-c04153c831b5
Fix source_span tests.

R=rnystrom@google.com

Review URL: https://codereview.chromium.org//712473002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/source_span@41614 260f80e4-7a28-3924-810f-c04153c831b5
Avoid instantiating FileLocations where possible in source_span.

R=rnystrom@google.com

Review URL: https://codereview.chromium.org//754463002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/source_span@41921 260f80e4-7a28-3924-810f-c04153c831b5
Add gitignore, status, and codereview files.
Update the pubspec's homepage link.