Merge pull request #83 from dart-lang/prep_publish

prep for publishing 3.0.2

Prepare to publish (#78)

Prepare to publish with null safety (#75)

Bump the `stream_channel` constraint so the lower bound is migrated for
null safety.

Prepare to publish (#54)

Added `strictProtocolChecks` named parameter to `Peer` and `Server` c…

…onstructors (#51)

Allows creating servers which are lenient towards misbehaving clients.

Add an unhandledError callback (#37)

Allows clients to handle or rethrow errors instead of silently swallowing them.

Bump to 2.0.10 (#39)

Version 2.0.9 was already published with no changelog.

Use Dart 2 constants.