Optimize surrogate decoding. (#894)

Use a slightly different approach to recognizing surrogate pairs, which can avoid some duplicate computations. Slight speedup measured locally when with `compile js` and `compile exe`.

prep to publish 6.0.0 of package:lints (#878)

Check for mandatory when using option (#871)

normalize the workflow files (#857)

update the topics section of several pubspecs (#854)

Moving fixes (#736)

update the pubspec version of package:collection (#730)

Merge pull request #579 from dart-lang/merge-collection-package

Merge `package:collection`

Merge pull request #405 from dart-lang/merge-typed_data-package

Merge `package:typed_data`

Merge pull request #404 from dart-lang/merge-platform-package

Merge `package:platform`