Deprecate new SourceFile() and add new SourceFile.fromString().
Improve tab highlighting.
Add SourceSpan.highlight().
Fix more strong-mode errors.
Allow some fields to be overridden in strong mode.
Fix the typing of FileSpan.{start,end}.
Add SourceLocationMixin and SourceLocationBase.
1.1.5
Fix another bug with external FileSpans.
Support external implementations of FileSpan.