Fix header for multispan with null secondary urls (#72)

If a multiple span highlight was created where all the secondary spans
had `null` URLs they were filtered out from the comparison checking
whether multiple files are involved.

This bug was caused during the null safety migration. A similar bug
previously existed if the primary span had a null URL and is also fixed.

- Take a nullable list in `isAllTheSame`. Avoid using `null` as a
  sentinel for no item seen, use `.first` and `.skip(1)` to compare all
  elements.
- Add tests for the case of primary and secondary spans with a null URL.
5 files changed
tree: de70a427aaea42a9832f708deef50b2a4b4aadea
  1. .github/
  2. .gitignore
  3. CHANGELOG.md
  4. LICENSE
  5. README.md
  6. analysis_options.yaml
  7. lib/
  8. pubspec.yaml
  9. test/
README.md

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.