Add support for highlighting multiple spans at once (#49)

This makes it possible for applications to provide additional context
in their span-based messages. Like the existing highlight format, it's
heavily inspired by [Rust's error messages][].

[Rust's error messages]:

For maximal backwards-compatibility, this doesn't change the
formatting of any single-span highlights and it uses extension methods
rather than adding new methods to existing classes.
11 files changed
tree: 6b412139013b0dedc61caf351b5a1f8e3237df16
  1. lib/
  2. test/
  3. .gitignore
  4. .test_config
  5. .travis.yml
  9. analysis_options.yaml
  10. pubspec.yaml

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.