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.


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
17 files changed
tree: f2276a3d3db521f3b1bf1fdbf3a5863782f6c859
  3. README.md
  4. lib/
  5. pubspec.yaml
  6. test/

source_span is a library for tracking locations in source code. It's designed to provide a standard representation for source code locations and spans so that disparate packages can easily pass them among one another, and to make it easy to generate human-friendly messages associated with a given piece of code.

The most commonly-used class is the package's namesake, SourceSpan. It represents a span of characters in some source file, and is often attached to an object that has been parsed to indicate where it was parsed from. It provides access to the text of the span via SourceSpan.text and can be used to produce human-friendly messages using SourceSpan.message().

When parsing code from a file, SourceFile is useful. Not only does it provide an efficient means of computing line and column numbers, SourceFile.span() returns special FileSpans that are able to provide more context for their error messages.