When clearing a field make the field state "does not exist"

This does not change the fact that the default value is returned when the field is cleared.

Tests for this in the dart-protoc-plugin package.

R=sigmund@google.com
BUG=https://github.com/dart-lang/dart-protobuf/issues/17

Review URL: https://chromiumcodereview.appspot.com//106663007