Merge pull request google#511 from efortuna/master

Modify pubspec to allow build package of 1.0 and greater

Merge pull request google#503 from davidmorgan/release-6-1-3

- Add `built_value_test` support for remaining built collections.

Merge pull request google#500 from davidmorgan/release-6-1-2

Release 6.1.2

Merge pull request google#496 from davidmorgan/release-6-1-1

Release 6.1.1.

Merge pull request google#486 from davidmorgan/release-6-1-0

Release 6.1.0.

Merge pull request google#467 from davidmorgan/release-6-0-0

Release 6.0.0.

Merge pull request google#463 from davidmorgan/bump-sdk

Allow SDK 2.0.0.

Merge pull request google#461 from davidmorgan/release-5-5-4

Release 5.5.4.

Merge pull request google#449 from davidmorgan/fix-build

Bump versions of build_runner and build_config.

Merge pull request google#448 from davidmorgan/release-5-5-2

Release 5.5.2.