Drop dependency on package:uuid (dart-lang/sse#32)

Switch to a v4 UUID. We were not depending on the specific UUID version
used and the random approach has a simpler implementation.

Copy a small implementation of UUID generation from
https://github.com/dart-lang/usage/blob/16fbfd90c58f16e016a295a880bc722d2547d2c9/lib/uuid/uuid.dart

Refactor to a top level method since it is only used once from this
package.