Testing only code (ie: matchers) should be exported through a specific library

[jakemac53]

See https://github.com/dart-lang/code_builder/blob/master/lib/src/matchers.dart#L5.

This causes problems internally because it prevents matcher from being a testonly target, which it should be. It also means matcher is a transitive dep of anything that depends on code_builder, which should not be the case.

This would probably have to be a breaking change though.